The latest Proteus 4 beta really offers the best experience by far...the interface is the one that complies the most with Apple HIG. It's got lots of different Contact list options (Show/Hide Status Text, Show/Hide Contact Images, Show/Hide Groups) as well as customizable status icons.
I really don't see how people can like Adium or Fire after using Proteus...but YMMV I guess.
Proteus 4 > ... > Adium X > Fire
The '...' indicates that a product could probably fit between Proteus and Adium X since there's such a wide gap in usability between the 2.
Fire is really the worst app in the world and I don't feel like talking about it anymore.
I realize this comment is like telling a mom her child is ugly or stupid and get knee jerk reactions from some of you but I'd like to be enlightened:
You see, I'm really scared of missing out on cool things other apps might offer...so, what does Adium X offer over Proteus 4 right now? ...other than being free. If it's the price, then for sure Adium X is the number 1 choice...but if you're not afraid to support an already small Mac dev community by paying for software that's good, why wouldn't you use Proteus 4?
Plus, where else would you get hilarious webkit themes like the Monkey Island Theme