Got to be Squadfather/QuakerOats/Sarcasm purely for legendary status - anyone worth their salt who's ever posted on here knows who he is.
In terms of "best" poster when it comes to putting their point across I would have to say Narcissistic Deity - that guy will pull out photos, spreadheets, graphs, etc just to prove that Dorian was better than Ronnie. Fucking relentless, but in a good way.