Firefox mainly, mostly for best add-on support, and lower memory consumption/better performances than Chrome (which comes 2nd on this point). Usually around 100 tabs opened, I use them as bookmarks, no way Chrome can handle this on my computer.
IE as 2nd, often having the two browsers opened at the same time (with about 10 tabs).
Sometimes using Opera as 3rd when I need a different proxy.
Only using Chrome when I have to (work), no way I could use this anti-standards memory eating thing. Especially when I already had to deal for long enough with the last browser who did the same (IE6).