What service are you trying to use? iChat is a client application that runs on OS X, and connects to services like AIM and MobileMe (plus other Jabber networks like GTalk, etc). Depending on what your needs are, that will better help determine what app works best for you.
Granted... I'm actually here to ask the same question, haha. I primarily use AIM, Skype, and facebook... and actually like the free AIM app for my AIM buddies. But I'd like to find one app to connect work with both of those services that plays well with the semi-multitasking/saved-state-ness of iOS4. I know that BeeJive was just updated, but haven't seen any full reviews on it yet. Is it that much better than IM+ or Meebo (or something else I haven't considered)?