Depends, if I'm trying to focus and feel energized... I go for the hard rock/alternative rock playlist: Nirvana, Danzig, Kyuss, Alice in Chains, Soundgarden, STP, GNR, Misfits, Ramones, Sex Pistols, Clash, Metallica.
If I'm trying to just find a stable groove and relax a bit, especially if I had a hard day I go with hip hop and trip hop... Massive Attack, Portishead, etc