The answer to this is two-fold. The site currently with the highest traffic for new US players is Global Poker. This answer spans both cash game and tournament traffic. There is action from the nano stakes up to NL2000 cash and tourney buy-ins from $1 all the way to $400.
If the best bonus is your thing, try playing at betonline.us. They have a 100% match up to $2,000 for all new players.