I use stock. I would use Opera Mobile because of how fast it is but it seems really flaky. The most annoying thing is that it doesn't make any attempt to scroll vertically when you try to scroll up or down a web page, it just takes the raw touch input so it will often scroll up or down slightly diagonally because people don't drag across the screen in a perfectly straight line. The stock Android browser has a filter that helps make sure scrolling that you intend to be only vertical is only vertical which by itself outweighs every single benefit Opera Mobile has.
