If you're doing dev work or just want to stay updated with the latest Google experience you should go with the Nexus S. Otherwise it will just depend on your carrier, off the top of my head I would say the best U.S. phones right now are ATT the Atrix, Verizon Incredible 2, Tmo LG G2x, Sprint still the Evo.

Personally I'm waiting for the Galaxy SII next month.