Well like most people I haven't been to enough to be able to fairly judge. The best stadiums in my opinion are ones where you feel a part of the city you're in, which is most easily reinforced by having a view of something cool outside the stadium. So the three best I've been to are:
1. Newlands Stadium, Cape Town (how beautiful is that?)
2. Skydome, Toronto (with the roof open)
3. US Cellular Park (Comiskey Park, as it then was), Chicago
The other ones I've been to are: McMahon Stadium, Calgary; Gottlieb Daimler Stadium, Stuttgart; Telstra Stadium, Sydney. I've been
outside of plenty more stadiums, but that doesn't count. Also not counting college/amateur stadia.
While not a stadium but an arena, I also love Madison Square Garden in New York. It has a sort of un-New York, dingy, retro feel to it that contrasts with the super-bright modern arenas (such as the ACC and Corel Centre/whatever-its-called-now).