Something easier to actually quantify accurately is length advantages, with height and reach. It's interesting that most people have Anderson, Georges and Jones as their top 3, and all three of those guys typically boast serious length advantages over their opponents--with Jones, it reaches the ridiculous level. I've stated it before, but there are a lot of people salivating over the prospect of a Jones/Cormier matchup in the future. If Cormier actually manages to make 205, Jones would boast over 20" in length advantage on him. A win over Cormier should not be a big asset to Jones' p4p resume, in my opinion. When you're an elite fighter and you have that big of an advantage before anyone steps into the cage...you should be winning the fight. Something is seriously fucked if you don't win it.
With size in perspective, I look to skillsets. Are they a single discipline fighter, are they well rounded? Are there any glaring holes in their game? A well-rounded fighter is going to get more credit p4p in my rankings than a one trick pony. Similarly, someone with a glaring hole in their game is going to have a harder time being considered elite for me.
Finally I look at their actual resume, the fighters they've faced and how they fared. What was the quality of their opponents, and how did they approach/execute the fight? When you have a guy who is beating the best at their own games, they shine in terms of pound for pound, in my opinion.
Aldo is on a 13 or 14 fight win streak, I think. He's been kingpin at FW for almost 4 years. In that time, he's beaten multiple elite fighters from higher weight classes who've moved down to beat him, he's beaten multiple fighters who were ranked as elite p4p in their own right. He's got no serious holes in his game...the biggest question one can pose is his conditioning, but it hasn't failed him yet. He's beating legitimate contenders that he gives up size to--generally the opposite of the Anderson/Georges/Jones scenario. I mean, Anderson gets credit for moving up to LHW and beating Irvin [who was actively addicted to narcotics during their bout, and who Anderson was actually bigger than] and for beating Forrest [who is considered a "huge" LHW, and yet their height/reach stats come out to be virtually the same]. Aldo accomplished more than that by beating Florian and Edgar, and zuffa didn't make any kind of a big deal about it. Georges gets enormous tuffguy credit for fighting Hardy [a guy who talked his way into a title shot and should never have been ranked top 10] with a pulled groin...Aldo just finished CSJ by kicking him with the foot he'd broken 15 minutes earlier.
I feel Aldo has less holes in his game, has beaten better competition--guys he often gave up size advantages to--and has an exemplary resume.
Again, you're welcome to your own opinions, but that's a bit of insight on how I formulate mine.