xScope because it's the perfect blend of lightweight and functionality. Used Dolphin in the past and it's good too but I'm sticking with xScope. A few weeks ago I reverted the phone to stock and used the stock browser for a week. That was an awful experience. The stock browser is better than no browser but that's about it. Of course it depends on how you use your phone, for me the browser is far and away the most used app.
I use DolphinHD because of all the features and it's quick user interface. I bought XScope to try it out and love some of its features like the one finger zoom, and it was really fast at some pages, but not the best UI.
Thanks, I decided to go with xScope. I like it best myself. I used the trial version for a while. It seems cleaner and faster than HD Dolphin which I used for quite a while.