Use ICQ, Yahoo and MSN.
ICQ is the best in my eyes, as it has a lot of useful options and most of people I know use it as well.
MSN is second best option, simply because it shows a message in low-right corner of the screen, whenever anybody signs-in, i receive a mail, etc. It shows the message even when i am playing a game and it doesn't interfere with anything or minimizes any other windows.
Use Yahoo simply because i have some contacts there now too.
Tried using AIM and was absolutely disgusted with it...
Tried Trillian, but it did not have as many options as other messanger's had. It's good, but since i can afford some memory to use 3 messangers at the same time and i restart my PC like once a month (so i don't have to load all 3 messangers that often

), I use them for more options instead.