Usability is what matters most. Does the browser function in the way in which YOU want it to? Is it well-featured?
Opera is pretty well specced and largely performs as it should. The only weak spot is the slightly fiddly cookie handling. Firefox is plainer out of the box, but very extendable and has absolutely brilliant cookie handling. It also has a greater range of appearances that Opera. IE, however, has none of this. It isn't anywhere near as adjustable, it is not as nice to use, the cookie handling is unbelievably bad and there's basically one boring appearance. I usually like MS products but IE (and OE as well) lets the side down.
They're all free and as has been clearly stated it's all a matter of personal preference, so why not download a few and see what suits you. It'll probably be Firefox because as usual VoG™ is right, He's always right he's irritating that way.