I'd say use your ear not scales, but playing around with scales helps your ear...
slayer are more chromatic than diatonic
anyway use the diatonic ones with minor 2nds in them, I always forget my mode names
playing by ear I use a lot of minor 2nds with major 3rds, it's pure evil :)
also try the half whole and whole half scales
the whole tone scale I'd say is more meshuggah than slayer, and is hard to use at first, or was for me, I'd accidentally make it sound major ish because of the major 2nd and 3rd