Currently using stock browser. Will likely switch to Fennec, when it is released.
Never installed Dolphin but, after reading through the features, I never really felt the need to. For me, the 'Windows' feature is just as easy as the 'Tabs' feature in Dolphin. I also think that in many cases, multi-touch requires more effort than double-tapping. Just my preference.
Long Live the iPhone Killer
I use the stock browser and have been tempted to try out Dolphin from time to time but to be honest the pinch-and-zoom feature alone hasn't been enough to make me switch. The regular browser just works so well and with how easy it is to pan around with the Droid's responsive screen and double-tap to zoom in, it's not like it's difficult to navigate on webpages. Far from it.