C Standard hands down. High on Fire, Sleep, Hatebreed, Nails, The Sword, Black Dahlia Murder, Ken Mode, early In Flames, Disfear, Coalesce, and others all inspired me to go for this tuning. E flat is a close second.
Drop C/D standard on a 6 string and Drop Ab/A# standard on a 7 string. Sometimes I like to mess around with open tunings like G-C-G-C-G-C-E on a 7 string and D-A-D-G-A-D on a 6 string.
I cant decide between E standard and D standard for my sixes.
Always A standard for sevens. B sounds too metalcore. A is nice and proggy. I actually think B sounds the most 'brutal' of all tunings, lower than B and you loose punch, higher and... well.. its higher. But I dont go for 'brutal'.
I'm always trying drop tunings but retune to standard 5 minutes later. Although I know some cool chords in drop tuning (search my thread history for my drop tuning chord thread), I still prefer chords in standard tuning. More options IMO.
It's usually standard just because i'm an ignorant prick and i can't understand my intervals anymore otherwise. But i'm fascinated by all kinds of tuning, especially open ones. I usually don't thrive below D tuning, by now at least.