It depends on what other services you use... ICQ, MSN, do you use any others?
Also, are you able to pay for a client?
I'd say if you can pay for it, you probably can't go wrong with Proteus, it's the most professional looking client out there, probably the nicest one I used, I really didn't like Fire or AdiumX, but I only use ICQ, so I went with something simpler in the end.