The first is very creative, and as a result the hardest to rationalise: imagine that all fighters are magically scaled to be in the same weight division - who wins? This isn't a very popular metric, but is one that is used by some in their arguments as you could say that if it wasn't for size, then Anderson would beat Fedor on the feet every time, or that if BJ hadn't been undersized compared to GSP, his skills would have let him control that fight. In this scenario Anderson Silva's striking is too good for anyone else in this list, and even should someone get him to the ground it's been shown that he can do enough not just to survive, but to win. BJ's boxing is a serious threat to most and his ground game is probably the best of anyone on this list should they all be the same size, but against Silva, his boxing will still fall short and against GSP he'll likely get pinned against a cage by a fighter who will still be physically stronger and a much more skilled wrestler. GSP has shown time and again that with his ability to get the measure of his opponents and then wrestle them through the mat for five rounds that should he get hands on his opponent, then they are going wherever he wants them to be – but first he has to get inside Silva and though GSP has good stand up, he’s not at Silva’s level and on the ground, after the Hardy performance it’s clear Silva or BJ at his size would give him fits. So when you have 3 guys theoretically the same size as Fedor, each of whom outclasses him in his given area, what do you think can happen? He can definitely throw a brilliantly timed, hand-pulverizing counter or pull off some sneaky sambo, but on a virtual, level playing field, he's going to lose to all 3 of his fellow Pound for Pounders nine times out of ten.
Assuming everyone is the same size
1) Anderson Silva
3) BJ Penn
The second, more popular metric is to look at their accomplishments relative to each other in their divisions. This is a better measure, but can be further confused by asking the question "if this is Pound for Pound", then what have they done outside their weight division?
In that case, we have to immediately move GSP and Fedor down the list as they haven't had any fights outside of their weight divisions in what we'll call "relevant history". Sure Fedor beat Aoki in an exhibition grappling match, but if someone wants to claim that as a pound-for-pound relevant bout, I will personally laugh in their face. Meanwhile both BJ and Anderson have fought above their weight division with great success. Indeed, the argument can (and has) been made that BJ could beat anyone at Welterweight except for GSP. Meanwhile Anderson has embarrassed two Light Heavyweights, including a recent title holder.
So the list using these metrics would probably look like this:
Accomplishments outside their weight division
1) BJ Penn
2) Anderson Silva
But why is GSP above Fedor in this list? Because of the third metric: dominance in their division. This is where I (and many others) feel the most weight should lie. First in this list is probably GSP as, since the Serra fight, he has never looked in trouble. No opponent has come close to finishing him and though he mightn't have finished fights emphatically, there's never any question of who won or lost. Add to this that there is no legitimate contender at this time for the welterweight crown and you have as close to the definition of divisional dominance as you can get. Second would be BJ Penn, as he is close to cleaning house in lightweight, which is a much deeper division - and again, no one has really had him in a bad spot.
Anderson and Fedor sit in an interesting position here. Fedor has dominated the division available to him, with wins over such "luminaries" in the past 4 years as Coleman, Hunt, Lindland, Choi, Sylvia, Arlovski and Rogers. He has been unquestionably rocked on a few occasions during these bouts, but ended all of them in fairly decisive manner. Meanwhile Silva has fought more than twice as many fights in that timeframe, many of them against top 10 competitors. The issue here is that in the eyes of some, he has failed to dominant in some of these performances. But I'm going to rule against Fedor here as his "dominance of the heavyweight division" has been against fighters who are at best, able to be described as on the decline, and at their worst, freak shows. Fedor’s management has locked him away from what is currently the deepest pool of heavyweight talent in the sport's history and without any real test against them, then Fedor will have to continue to sink in these rankings as no one should seriously rate Werdum or Overeem at this time.
Dominance in their division
2) BJ Penn
3) Anderson Silva
So, does anyone have any observations about these 3 lists? I'll point out the obvious one. That in each of them the 3 top guys have all switched positions, so that they all rate first, second and third in the respective metrics. Meanwhile Fedor finishes in fourth place every time. In fact, I'm willing to go out on a limb and say that by the end of this year, Fedor will not feature in the top five of any realistic pound for pound list as the quality of the competition he will face cannot stand up against guys who are testing themselves against fellow top competitors regularly. Sure, he will in all likelihood go undefeated, but when you last match against a truly relevant opponent was back when Pride was still around, you have to do more than fight Werdum to get back into contention. At this point I'd say that the winner of Lesnar/Carwin is more deserving of Fedor's spot on this list than he will be, unless he somehow beats Werdum so badly that we all forget to play six-degrees-of-MMA-separation and look at Werdum's history and those of his wins and losses and realise just how irrelevant that fight really is in the heavyweight division.
Before anyone jumps on me for it - I'm not a Fedor hater. I love watching him fight because it always turns into some bizarro world where the most outrageous things happen and I find that to be outstanding. But I can't and won't take him seriously as a top heavyweight contender, let alone a pound for pound contender until he takes on some real competition again.