I would say firefox is the best! It is the fastest on almost any operating system, even Mac - I know because I have it and the tabbed window system is amazing! . It also has better support for things like javascript and stuff.
Opera for me. I also use Firefox, IE7 and Safari... since when I'm writing sites I like to make them suit each browser individually (at least when I'm writing a page for my school's intranet...) We use all the browsers... users get a choice of which one.
I hate opera (continuing my prior one) it is standards compliant, but has too many security vulnerabilities that apple refuses to fix for older machines! (really nasty "smiley face" here)
I uses firefox most of the time because of its wide range of plugins for web developers ( but sometimes IE7 to check if my site displays alright. Opera is cool but rarely use it.
I think the desing of IE7 is good but it's still so vulnerable.
Firefox is nice but it has a few options if you compare it with Opera.
So i think for now, Firefox rulles ( You could try testing Firefox 3 Alfa here ).
I too like Firefox but I find IE7 has made up lot of things which IE6 was lacking like tabbed browsing.
I have also observed that IE7 loads much faster than Firefox, may be because of its nativity in windows.