XScope is pretty awesome. Dolphin (HD / Mini) are up there with it too though. I had some troubles with certain Flash sites and didn't like the way XScope let you zoom out as far as you wanted. I prefer the browser to stop zooming out at a certain point.
I also really liked how Dolphin HD had the side-bar features which work really well on a tablet. You can just drag the page to the right and the bookmark bar will show up letting you quickly jump to a new page. More than anything, I'm really looking forward to Honeycomb's new browser... it sounds like it should have most of the great features I'm looking for in a browser and it looks good (XScope and Dolphin don't really blend in well visually).