Not a dumb question at all, but certainly one that's likely to spark a lot of debate, as everybody has their preferred messenger system.
I'm quite fond of the iChat interface and design, and it does offer certain advantages over AIM, in that it can connect on a local network, such as an Airport wireless link, without needing to access the Internet.
However, for my instant messenging I use an app called "Fire" which offers support for AOL, MSN, Yahoo, ICQ and IRC. Others on the site here highly recommend Adium, and I know there are about 10 or so other messenger apps that are well liked here. So, it's really a matter of your own needs and tastes.