For me it would be Trillian or Adium... that's all I need basically. It's interesting to see that both of these aren't in the list. Whevener I'm on windows I tend to use Trillian because I have lots of friends that prefer different kinds of IM services (mainly MSN though). Plus, I love it that Trillian doesn't have any ads and it's completely customizable and light weight.
On the Mac, Adium is probably the best, because it supports all protocols and along with that looks cool, and if you don't like the way it looks you can change it in an instant, which is the best part. So if you really love the iChat style you can get that style too... and there are some other cool ones out there that are just awesome.