When playing Metal or Rock, my best advice is to use Pentatonic and Natural Minor scale. It always sounds catchy, Kirk Hammet, Marty Friedman and Dimebag use them a lot, but it sounds redundant if you use it too much.

Harmonic Minor sounds best when played fast and you want to sound a little classical, It sounds exotic. Its good to play it with arpeggios and sweeps. Yngwie Malmsteen and George Bellas use it a lot.

Apart from those two scales, the following are also great for metal and rock.

Dorian

Ionian

Aeolian

Lydian

Myxolidian - Satch use this often

Bebop Major - a little jazzy, (fussion)

Egyptian scales ! - Ritchie Blackmore use this sometimes.