When I first tried to connect my old Sony Ericsson k750i to instant messengers, I came across fring. I later re-discovered fring when I had windows mobile devices and found it to be the best solution I came across so far. Then my friends started to have iPhones and I introduced the to more fring.
Eventually I got myself an Android, but there was no fring.
After searching for a viable replacement, I finally found Nimbuzz. Nimbuzz is like fring in the way that it connects to pretty much anything, that it has it's own chat protocol and that you can both type and voice chat.
The beauty of it was that it also connected to facebook chat, something I didn't see until then. I also found out that it has got a pretty cool online interface and you can manage your connections through that. In theory you can set it up online and then connect through your phone using your Nimbuzz username and password and everything populates itself automatically.
Having your instant messengers available while you are out and about is one of the most important features for me on a smartphone and it's difficult to find a better solution than Nimuzz.
Even as fring finally released their Android app, I was amongst the testers, I found Nimbuzz to be the superior app. Not only because of the many, many bugs in the beta version. Maybe it was because of it's online access, maybe because of the facebook protocol - I don't quite now anymore.
I still have both fring and Nimbuzz installed, mainly because I don't know if you can make voice calls to normal numbers using Skype over Nimbuzz. Something that definitely works now on fring. Also fring does look kinda nice, they certainly spend some time making it look other than the standard iPhone / Android apps.
For now, my vote for best Instant Messenger goes to Nimbuzz