Good thread.
1. Top billing has to go to the Big Show. His legacy will be great. He has many accolades and has enough charisma to forever last in the minds of two different generations of wrestling fans, but it's time to go. Un-Bo-Lievable Jeff said that he can still put people over but I disagree. Fans are legitimately tired of Paul Wight: it's not his gimmick, not the fact he's a heel (or is he a babyface now? He changes every month so I lost track), it's the man himself. Literally nothing that they do with him will make people interested. I was at SummerSlam this year and let me tell you my entire section (which was a 100 section) left for beer/bathroom as soon as they heard his theme. He has nothing left to prove, and unfortunately for him nothing left to offer.
2. Mark Henry - I wasn't really ever on the Hall of Pain Train, but that run gave me some entertainment on the mic. He has never had a good match in his life, and is by far the worst wrestler who fell victim to The Streak. At this point, maybe he can put some people over, he still gets a good reaction is obviously a big guy. At 44, I simply feel like he's nothing more than enhancement talent. That's fine, but if I'm him I probably would rather retire with some pride than be relegated to a joke for several more years.
3. Daniel Bryan - I never would've even thought of him, but wweguru makes a good point: no matter how much DB and WWE Universe (including myself) chant YES! YES! YES! It looks like his body is saying NO! NO! NO! Just like Kobe Bryant in the NBA we've seen back to back year-ending injuries occur to a top guy who has quite clearly dedicated his entire life to this craft. Both started so young and fought so hard get to the top, but safety should always come first. For Edge it was a no-brainer, and although Kobe is trying to have "one last year" I don't think Bryan should unless he tailors his move set. Since Bryan has gone on record to specifically say he would not tailor is move set around safety, I think it's best to ride off into the sunset with the ability to walk.
4. Undertaker - He would've been number 2 if I didn't attend SummerSlam this year. I haven't been interested in the idea of Taker match since WM29, and I haven't been entertained by one since WM27. He's looks so old and terrible, he moves so slow and feeble, his gimmick has now been of someone old and "losing it" for years. Going to SummerSlam and seeing how many people were specifically there to see him and so emotionally and financially invested in him made me realize he's still a cash cow, even in his old age. I'll never forget an interview he did in 2003 where he criticized Hogan and Flair for "hanging on too long after their primes" and vowed to go out on top so that people will only remember him in his best shape. Fast forward 12 years and he's literally the face of the "Holding on Too Long" club.
5. Natalya - Another controversial choice, Nattie Neidhart is the best female wrestler in the world today. She was also the best female wrestler in the world in 2010 and all of the years in-between. However, I don't think any WWE Universe member would put her in their top 3, which is a direct result of how poorly her entire career was booked. The first ever 3rd generation diva; with so much charisma, legitimate ring ability and a pretty face should've been a golden goose for WWE. She should've been the Uncle Bret Hart of the Divas division, receiving mention among Lita, Trish, and Moolah. Instead, they've made her mostly her husband's valet and gave her a farting gimmick. I don't think she should retire from wrestling but she should definitely retire from WWE. She's about 33 so she probably doesn't have many years left if she's going to start a family with Tyson (and women generally retire in their mid-30s) so she should spend those years getting the respect she deserves. Tyson deserves better too, maybe move them to ROH or New Japan or something, but this whole shit of her "missing out on the Divas Revolution" is disrespect: she should be out there battling with the 4 Horsewomen (Charlotte, Becky, Sasha, Bayley) every night.
As for Kane, Sting, HHH, and R-Truth: I see no reason for any to retire. Kane is still very over, and has been really good in the authority although I do think his ring work has been lackluster for a couple of years. Sting, the Icon, is obviously massively over and provides many dream match opportunities. He still looks the part and can go in the ring so really the only thing people have to talk about is his age, which shouldn't be the end all be all. Hunter is probably in the best shape of all 4 of these men, and with him only working once a year now I can honestly see him performing at a high level well into his 50's. As for Truth, he's a great enhancement talent that still provides a lot of entertainment and can put on a good match in the ring.