Windows Messenger is probably the best of the three biggest IM clients, but has the least number of users. AOL has the most users but I'm not a fan of their software. Unfortunately most of the companies I deal with have AOL IM so I have to use it as well.

I have IM accounts on all three services and use Trillian to manage them. Trillian is a freeware client that allows you to connect and converse with all three services (www.trillian.cc). It does not have the extra features such as voice, but it is great for having all three running at once and combining the buddy lists from all three.