Firefox.
I've tried Chrome, but last I checked it didn't have a good adblocker. Also, I much prefer having a seperate search bar. I have quite a few site-specific searches, like Bulbapedia, Smogon, Arch Wiki and AUR, Wikipedia, YouTube, and more. If I used Chrome I'd have to set up and remember codes for them all, with Firefox I get a visual menu.
In the past I've had a problem with Opera with typing in forms, especially on Facebook, being highly unresponsive. Far from the claimed speed, that made it feel glacially slow.