The best in the world in real life, absolutely not. Within kayfabe, however, the World/WWE/TNA Champion is the best in the world. That's what being the top champion in the federation means. Why put the belt on someone if they are not meant to be the top guy and the main attraction of that particular event? It devalues both the belt and the wrestler if a world title holder does not close a show.
For a prolonged period of time, yes. I don't think the argument can even be made that Punk vs Jericho at either Wrestlemania or Extreme Rules should have closed the show over Rock/Cena, or Cena/Lesnar. In both cases, you simply had bigger stars in more heavily hyped matches, while neither of the championship matches(both of them) had been presented with half of the fanfare that the Cena matches had been given.
And there are times where a belt is put on a guy as a test run, to see how they do with the opportunity. Jack Swagger in 2010 and Daniel Bryan are recent examples of this, relatively unproven guys within a main event landscape who were given title runs. But sine they were champion, should they have been main eventing PPV's? Absolutely not. Those places go to the bigger stars who are more established and have shown they can draw.
There is an inherent danger of not having your top champion in the main event slot for a prolonged period of time, however. It's been 6 PPV's since the WWE Champion, CM Punk, has main evented a PPV. The WWE Championship is generally regarded as being the top belt within the company, yet Punk hasn't closed a show in 2012. In those cases, it's saying that certain people, in this case, John Cena, is bigger then the championship. And by putting a titleless Cena in the main event against men such as Kane, John Laurinitis and Big Show simply points to Cena and what he's doing as being more important then anything going on in the company.
In this scenario, balance is the best thing possible, for all involved. Cena is undoubtably the top draw, but that doesn't mean people are necessarily purchasing a PPV first and foremost to see him face Kane, a GM, or Big Show. In order to avoid devaluing the top championship, it needs to be presented as the focal point of shows more. It sends the message that the champion is important, the title is important, and the title defense is important. It's not as if it's being pushed down the card, as it's still been in the semi-main event slot, but there needs to be that balance where the champion and the defense of the title needs to be in the main event slot. Right now, WWE isn't finding that balance, and the "B" level PPV's are suffering somewhat as a result.
They could put the championship on one of the guys Ryback has been squashing, it would still need to go on last unless there was a HUGE dream match on the same card.
Would you really tune in for a joke of a champion to be main eventing a PPV? I know I wouldn't. The man carrying the championship is more important then the championship itself, and in this scenario, it wouldn't take much of a match to top a jobber with the World Title championship match. You'ld have bigger issues of the title and the PPV both being devalued if you stuck it in the main event in this scenario.
Cena facing Big Show or Johhny should not have gone on after the world title. Punk is (kayfabe) the best in the world right now and he should be booked as such. I won't even START on Sheamus opening shows.
Again, it's about balance. In the cases of both OTL and NWO, the angles with Cena/Laurinitis and Cena/Show were the biggest ones on TV heading into the PPV, which creates a bigger issue: Do you take the biggest angle and stick it in a lesser spot on the card just because the title should be presented as most important? In these cases, it's on creative for not taking what's been a compelling dynamic between Punk/Bryan/Kane and AJ and not presenting that as the main event in segments on Raw. Since the feud has been booked as a secondary one to the Cena feuds, it's hard to justify putting it over the Cena matches on PPV.
But again, it's about balance. The WWE Title has been devalued somewhat during Punk's current reign as champion, and it's not because Punk is champion. It's because Punk as champion hasn't been presented as the main event on TV in some time, and hasn't been featured in the main event in 2012. He has in the past, so there's no reason he shouldn't be currently. I understand big matches such as Cena/Rock and Cena/Lesnar bein given TV main event time to build towards a PPV main event, but Cena/Laurinitis or Cena/Big Show? Not so much.
Sometimes it's as simple as that. The way the feud is presented on TV is often going to determine it's place on the PPV card. Feature the champion and the championship feud in the main event on TV more, and it's much easier to put it in the main event spot on PPV.