I agree it's a really tough topic. I think you have to exclude anyone who hasn't been around for the entire decade personally, though others may disagree. You may feel that where Orton eventually got to makes up for where he was in the first half of the decade, but it's my opinion that it does not. I'd also eliminate the likes of Austin and Rock because though they had massive decades, they ran between two decades (as in 1995-2005 sort of time). If you ignore everything they did before 2000, you can't really say they were wrestlers of the decade. Brock Lesnar may have made this list had he stayed.
I'd say the top candidates are Triple H, Shawn Michaels, Undertaker, Angle, Jericho and John Cena.
Triple H has possibly been the WWE's most important wrestler for the entirety of the decade, minus his injury time. Cena has been their most important wrestler for the time he's been at the very top, but that isn't as long as Triple H has been. He began the decade as WWF Champion, was the wrestler of the year that year in my opinion, having his ongoing feud with The Rock and performing at a very high level. Then he moved on to his feud with Austin which was good, though a little rushed. Then there was the 2 Man Power Trip which I personally enjoyed before he was injured. He came back as a face for the first time in quite a while, won the Undisputed Title, was later handed the World Heavyweight title, then moved on to the eternal feud with Shawn. Him going over every former WCW talent (and RVD) was a bit of a dull spell, but Evolution was a good group. He put Batista over big time, which I didn't care for as I can't stand Dave, but hey, you can't say he never puts anyone over. The reunion of DX was good at first, though eventually got a little bit hollow. Still, he was on top, feuding with the likes of Edge and Orton. Then came another big solo run on Smackdown and him putting over Jeff Hardy as well. Then he had what was in my opinion a really great feud with Orton.
All in all, he's held many, many world titles, been involved in three huge groups (McMahon-Helmsley Faction, Evolution, and DX), feuded with every main event talent going, missed probably only a year to a year and a half due to injury, helped make the likes of Batista and Jeff Hardy into legit main event talent, taken two tap-out losses at Wrestlemania (something people say he would never ever do), been a top-level heel and a top-level face. Bottom line is, he's been pretty reliable in my opinion. From 2000 to 2010 he's brought his A-Game fairly consistently, perhaps getting a little lazy when he monopolized Raw, but still, I think he's a better in-ring performer than John Cena.
Shawn Michaels gets a nod from me not for his importance to the company, but for his wrestling ability. In the ring he may have been the best wrestler not named Kurt Angle this decade. I feel I'm contradicting myself by giving him a nod when I penalized Orton for only being at the top for 4 or 5 years, but I think Michaels' sheer magnitude makes up for it. His first match back was much better than I think anyone could have predicted. His title run felt like it was a token of thanks rather than a great decision, but he's gone the rest of that time as a main eventer who has never won the title, and yet he still gets the huge pops and he wins almost all his matches, so that's pretty special in my opinion. His matches in 2003 were as good as anything he did in the 90s, and I'd say thanks to his experience he was actually better than he'd ever been as we moved through the decade. Shawn has probably given us a match of the year candidate every single year, against the likes of Triple H, Chris Jericho, Kurt Angle, John Cena, Ric Flair and the pair of classics against Undertaker. His feuds have been really good because most of them weren't based on championships, and the skill of putting together a non-title feud has seemingly been lost over the course of the decade.
Shawn has basically been a phenomenal in-ring performer for the whole decade, though he hasn't carried the company, been the top face, been a heel for more than about 2 months, or won many titles. He hasn't been as important to the company as some others, though he put on wrestling clinics.
Undertaker has had a crazy decade. From the American Bad-Ass and back to the Deadman, and then some strange hybrid. He was a fairly decent heel as well as perhaps the most popular face in all of wrestling. Nobody gets a reaction during their entrance like Taker. His presence is just undeniable. He's also surprised me by picking up several world titles. I thought he'd go the Shawn Michaels route and perhaps get one token thank-you win, but then go mostly without it, but ever since he beat Batista for the belt he's been collecting gold left right and center. He, like Shawn, has been one of the best in-ring performers this decade with classics against a plethora of wrestlers. He's also had some duds against the various monsters that have been brought in, but that's not his fault, so I won't punish him for it. I think the fact he's held multiple world titles and has seemed to carry Smackdown gives him an edge over Shawn, though the length of time he's been away makes him fall short of Triple H.
Taker is the last legend who can be considered truly active (Flair and Hogan don't count) and he may be the most popular wrestler of the decade, as well as one of the top 3 in-ring performers. He's been involved in some of the top storylines, and nobody gets the same treatment as him, making his appearances truly special... but he's missed a great deal of time due to injuries and simply wanting time off, which while it may not be entirely his fault, I'm going to say means he's been a little less reliable.
Kurt Angle has spent the decade proving he's the best pure wrestler alive, and perhaps ever. Nobody can match his technical skill. His ground game and legitimate background has given him the reputation of being a truly dangerous wrestler. He also learned the intricacies of pro wrestling far better than I ever thought he would. Things like how to cut a promo (he's one of the most underrated in this regard), and everything beyond pure technical wrestling. He won every single title he had a shot at, and most of them within his first year of wrestling. He went from the goodie-two-shoes mid-card and main event heel, to Team WWF's top face against the Alliance. His feud with Austin made him in my opinion. If you took the matches of everybody on this list and compared them, Angle probably wins. But he also rose to another level over time, becoming a legit main event talent that got real pops, as opposed to when the audience didn't seem to quite accept him early on. He's been a top heel and a top face. Perhaps most importantly, unlike the others on this list, he's done it in two different wrestling companies. His work in TNA has been just as good and at times better than anything you'll see in the WWE. His feuds with Samoa Joe, Sting and AJ Styles have been some of the best of the decade. The headbutt heard around the world was amazing. He's also been putting over a long list of young talent in TNA, something many WWE wrestlers are guilty of not doing.
Angle is the best in-ring wrestler in the world. He's done everything you can do and in TWO companies. He's stayed relatively injury free despite that chronic bad neck and pain killer addiction. He's helped put over a lot of talent. I'd say he's fighting Triple H so far on my list. I'm not quite sure if he's shifted as much merchandise or received as much of a reaction from crowds as HHH... I'll come back to it.
Chris Jericho perhaps the most versatile performer in the world. For a brief period in 2000 he may have been the most over guy in the company. His mic work was hilarious, and he was putting on clinics with Benoit, Angle, Eddie and Regal. He was the first ever Undisputed Champion. I think he got a little bit boring when he turned face again after his feud with Christian (which was really good), and not long after he left wrestling altogether. Until then he'd been a solid but not stellar talent, but all that changed a few months after his failure of a return. His feud with Shawn Michaels was a-maze-ing. His mic work was on another planet. His wrestling became smarter. He used to be known for having tonnes of exciting moves, but in the last few years he doesn't actually do much in the ring, but everything makes sense, he knows what to do and when to do it. He's been in charge of his own feuds to an extent, and he's had a lot of really good ones since his rebirth. He also helped bring back some semblance of importance to the tag team titles. Admirably, he's put over EVERYONE he's worked with, and he's pulled triple duty at times, being on Raw, NXT and Smackdown. That's just unrivaled dependability. However, it took him until nearer the end of the decade to find himself, as as amazing as his few years have been, they don't earn him top billing for me. His time off and his duller face moments hurt his chances, and though putting people over is a good thing, you can argue he hasn't won enough big matches, but that's purely subjective.
Jericho has been a god among men, but he hasn't done it for long enough.
Finally, John Cena. Say whatever you like about his wrestling ability, he has put the company on his back and been this decade's Austin/Rock hybrid (not as big, but in terms of his position in the company), and it appears he'll be sticking around longer than both of them. He's shifted more merchandise than anyone, even Rey Mysterio and Jeff Hardy. He is utterly adored by children, and children are the most important fans to win over in wrestling because they buy the merchandise, they get their families to take them to shows, and they will grow up to be loyal fans. He's been booed by men all over the world and yet he truly seems unphased. He's released a poor rap album, made more public appearances than any other wrestler, and has a mediocre film career going. The quality of the films is irrelevant, it's the fact he's making them year after year and still wrestling, unlike The Rock. He truly bleeds WWE. Perhaps never has there been a better company man in the industry. I admire his professionalism, and his ability to weather the storm despite everything thrown at him. He's also proven to be a very fast healer, coming back from a couple of injuries quicker than expected, and his returns have gotten huge pops.
It's tough to knock a guy as devoted as Cena, regardless of his sub-par ring-work. He's the top man and has been for a long time. He doesn't appear to be slowing down at all, and he's bringing in the money for the company. You could argue he's been the most important wrestler this decade. But does the fact he's only been a face mean he can't compete with Trips and Angle? Let's take it to a final vote.
My three finalists would be Triple H, Kurt Angle and John Cena. The first two have been more versatile than Cena, but do you need to be a top heel if you're a really good face? If versatility is an important aspect of this decision making process, I guess Cena is out, but I don't think he should be punished for it, and I think his merchandise sales and the fact he's still wrestling despite all his other commitments gives him an edge over the other two. Triple H will always get the "well, he's doing Steph" criticism against him, and while Angle is a better wrestler and more versatile performer than Cena, and isn't doing Steph to our knowledge, I don't think he's quite hit the heights of Triple H and Cena in terms of audience reception.
If push comes to shove... I'd say Cena has been the most important wrestler in the industry this decade. For better or worse, he's created more buzz than anyone else. I mean to have so many people in the IWC bashing the hell out of him and still be on top and doing fine, that says something. All press is good press right? He IS the WWE. We may never have seen a more dedicated performer, and to me, that makes him the wrestler of the decade.
Essay over.
