Opera on WM6 is very impressive at the moment, but on a mobile device I have to say that running Mozilla on the N800 was the best I've ever seen. The N800 is a little underpowered for large site renderings, bit the browser handled everything I threw at it
In the UMPC market the second generation devices like the Samsung Q1Ultra are really quite usable. Faster CPU's and better displays. Of course, the Shift is the one that'll set a new benchmark for them...