So far I've had the stock (obviously), Dolphin (haven't tried HD yet), Skyfire, Opera, and xScope.
Stock - is, well, it's stock. Nothing special but gt's the job done if that's all you're looking for.
Dolphin - was fine but I'm not into the use of all the gestures when other methods work faster for me for browsing. You don't have to use the gestures of course but it just didn't solve all my browsing needs.
Skyfire - made some sites available that use flash that other browsers just couldn't do. But I also found it buggy, a resource hog, and had a fairly large footprint.
Opera - fast indeed but I'd noticed that it wasn't able to properly operate some websites. Touch a hyperlink to select it and the whole screen highlighted or a link I wasn't trying to was selected.
xScope - worked the best for me so far. Fast, maybe not the fastest but fast enough. Has tabbed browsing. Can select whether I want a website to display as either Android or iPhone mobile platforms, or as seen on a regular desktop. And it's a fairly light file.
In the end you're best bet is to try them all to find which best suits your needs. YMMV.