I've been using Opera as my main browser for years, but also uses Chrome and Firefox.

Chrome; very fast, but crashes too often, and uses a LOT of memory (and messes up my Task Manager)

Firefox; lots of interesting add-ons, but slow and memory-hungry (and too often updated, with too many unstable updates)

Opera; very fast, very stable, best with regards to memory use. However, has problems with a few web sites (not compatible, maybe because they haven't tested "leading edge" technologies on Opera ?)