There was a time when Opera was it, then it was Firefox, then SRWare Iron, then Firefox... but these days it's Chromium. Certainly not religious about my browser, and may well switch again. Doubt that I'll go back to Firefox, though, but maybe another Chromium flavour or Opera.
The 'why' bit: I had too many unpleasant experiences with Firefox, mostly regarding extensions, and I was tired of having to restart the browser every time I installed/uninstalled extensions. Maybe that's changed now, but put me off. Firefox always seemed slower to me, both startup and browsing. I don't have any facts to back it up, but it just felt that way...
For Opera, too many pages looked a bit wrong and I was not impressed with the lack of 3rd party extensions. Now that it's changed, I'll give it another run for the money (it's originally Norwegian, like me, so there's a patriotic reason as well
Chromium: Just works. I had a problem, which was easily fixed by deactivating an extension (no restarting this or that). Would like to be able to modify the context menu and a few other things, like in Opera, but don't want it enough to not use it.