AIM 5.5 w/ AIMutation or DeadAIM. why? because its one of the few recent AIM versions that doesnt look too bad and functions the way i like it. doesnt randomly crash either. nice, fast, and smooth for me. plus, there are several good addons, like middleman, messenger mate, etc.
trillian was kinda complex and there were too many options, gaim was too...crappy looking and unstable for me. never tried miranda.