Best/Worst is relative. I stopped using IM services when I didn't have a computer of my own for a period of time, and this was pre-smartphones as they currently exist. I used Meebo for a while, but the interface just started getting on my nerves. (Pre-Trillian as a service too.)
I went back for a while with the advent of Beejive (iPhone), IM+, etc. But Facebook won for me because of the sheer number of people that use it, and the identity problem. I won't lie, it's real nice having my friend's actual names in my list (and I don't have to constantly manage their aliases from system to system).