An iPhone without a doubt. Its not the most technically accomplished, as it doesn't support flash, which other browsers do. The screen doesn't have the highest res, or the fastest connectivity. It doesn't matter one jot as the browser is frankly amazing. Its fast, quick, and intutive. Its really how a mobile browser should be. Opera Mini, Opera Mobile and netfront do passable jobs sometimes but they just don't compare.
On paper it doesn't stack up, but try one. I've used so many mobile browsers, and in the past I would have said Opera Mini (PIE is just so rubbish), but having spent a lot of time with Safari on the iphone, its impressive.
Put it this way, my wife likes to check websites sometimes in bed, and she normally dumps the phone on me and asks me to look them up for her. With the iPhone she'll happyily navigate websites her self, its that easy and makes sense to her. It just works.
As phone though the iPhone arguablly is quite poor, depending on what you want.