First -> Yes Man
Second -> Caesar's Legion
Third -> NCR
Now -> Mr. House
Siding with Yes Man was very cool, specially because you would Rule the Mojave, and because you had the option to choose what to do with all the tribes. Not like in the other playtroughs, where you HAD to DESTROY the Brotherhood of Steel. (Dude, they're just awesome)
Caesar's Legion --- well, being evil is just sexy, you know. Abusing, murdering, stealing innocent people... that's sweet.
NCR - oh, I don't know. Thinking about it, it seems to be the worst playtrough in the game. Not too funny at all, IMO. Actually, it is pretty stupid, because you don't even need to TOUCH the platinum chip in order to complete it.
Mr. House is a very interesting option specially because you can learn a lot of things about the Fallout World and the Mojave with him. And it is also an amazing option for players that want to have a Merc character (not too evil, not too good, doin' everything for money and power).