Firefox - I have used it since my Windows days, and since it was a 0.3 release back in the Firebird days.

For me it's the most stable, has the best functionality (ad-block actually works without downloading ads for example) and the nano-second speed gains you get from Safari or Chrome for me don't justify the switch.

I quite like Safari, but every now and again it will do something I don't like and I fond myself back on Firefox.