I've heart that if domain is country specific site usually ranks high in regional search. Which domains are good for US market ? I've never seen .us domains ranked well in google usa But for example co.uk are well ranked in google UK regional search.
It's difficult to answer in non trite way, but .com ofcourse... best for the worlds biggest internet market and good for the others... what to say more?
every major website in the US is .com, so your gonna have trouble trying to improve your ranking with a .us site!