riffing tends to be a mix of locrian, sometimes changing the minor 3rd to a major 3rd, which is probably some mode of harmonic minor, I also use half whole and whole half and sometimes the whole tone scales
basically what ever sounds good, evil and heavy
melodies and clean stuff seem to always be in natural minor, shredding is any minor key
for some reason I've started doing pentatonic "bluesy" shred, I only started focusing on pentatonics more to learn the whole of the fretboard properly, I only learnt what double stops are last night :)