What is the best spec for a rogue?

Answer 1

Depends on what you want to do. There are different specs depending on the situation (ie raiding, PVP, Soloing and dailies). This link has a listing of various rogue builds:

http://www.wowwiki.com/Rogue_builds

Answer 2

It depends on what you prefer. Combat if you hate the stealth feature. Assassination if you prefer stealth.