It really comes down to your friends.... If more use AOL or AIM, then thats what you should use. Same goes for ICQ. Ideally there should be a "standard" and all the IM services could live as one (fat chance since AOL is a monopoly).
Personally I use AIM, but only because most I know use "it" or AOL. I also use Jabber since it works with ICQ, MSN and Yahoo. At first, Jabber also worked with AOL but to no surprise America Online Corp pulled the plug 🙁