The term "pound-for-pound" was first used in boxing as a way to compare highly skilled boxers who were separated by too many weight classes to actually fight each other. It was a way of saying, "Well, these two boxers will never fight each other. But let`s compare them based on their respective performances within their divisions. Who do you think has been more impressive?" Of course, this was long before there was MMA and long before their were women competing in boxing and MMA.
There seems to be a lot of (young?) sherdoggers who get really hung up on this term... it seems like they interpret the word "pound" too literally. P4P Best is simply a way of comparing fighters who will never actually be able to fight due to great discrepancies in weight or - now, in this era - differences in gender. In other words, who looks most impressive within their division and in regards to the opponents they`ve faced? But still, you get some people who insist that men and women should fight each other ignoring the fact that men`s bantam weight and women`s bantam weight are two different divisions (if you are still lost on this point, read the above paragraph again)...and bringing this point up with them is like arguing with a small child (i.e."Nuh-uh!!!!! Pound-for-pound means exactly the same weight, man or woman! Pound means weight! It means men and women should fight! No way a woman is beating a man in a fight! Pshaw!"). These childish posts are made by people who are either deliberately ignoring what the original intent of a pound-for-pound list was/is...or they are just very insecure when the topic of capable, strong, and confident women comes up. They seem to be most afraid of Ronda Rousey.
When comparing the different divisions within the UFC, and the best fighters within those divisions who can never fight each other, you need to look at their record, their performances, and how well they do in comparison to the rest of the field. Ronda Rousey has a smaller number of fights than most of the other champions in the UFC (close to the same number as Dillashaw and Weidman, though). But, she is undefeated, with all finishes, all but one finish in the first round. She didn`t start by fighting cans and padding her record in local promotions like most current champs did. She beat Top 5 ranked opponents even at 145 in her first three fights. Based on this evidence, no other current champion is as dominant as her - except maybe Jones - and the only reason not to rank her #1 on the P4P Best is that she just needs more fights. In my opinion, she`s number 1 or number 2 on any pound-for-pound discussion.
Jarl