Just curious to get everyones opinion. I am currently using Avant.
Avant is a tiny bit faster and is very compatible because of the IE backend, also has better UI and excellent features builtin. Of course, it also has all the IE security problems. Many people don't like the spyware it offers although it is not installed by default and only moronic users manage to put the RoboForm on their machines.
Mozilla comes with builtin mail/chat/news, it's open source and cross platform. It is secure and you can get tons of extensions and plugins. Firebird version is very fast and lightweight although still too beta for my tastes. UI is clumsy IMHO and it was a pain to tweak the thing to my liking.
I personally use Avant at home for compatibility (Opera all the way otherwise) and Mozilla at work.
I have not used Avant before but I have used Opera and Mozilla 1.6. The best browser has to be Mozilla Firebird 0.7. Check it out