MSN for me.  I use my webcam fairly regularly and Yahoo's webcam pops up in a seperate window.  Aim's webcam is just crappy from what I remember like a year ago, might be different now though I dunno.   Anyway, MSN has the best setup IMO, it's clean and simple, but with various features that aren't too much of a confusion while sometimes useful.  Version 7 just came out also.   Plus, u get to use yur own custom emoticons if yur into that.