i prefer undeath / zombies over demons so
death knight for me.
a batshit crazy warrior is pretty badass aswell.
I don't know
Warlocks are definitely more bad.
DKs are also bad but plagues/diseases are just not as evil as demonic soul consumption...
In either case, that wasn't even Gul'dan. He corrupted the orcs, yes, but all the scheming was done by Kil'jaedan and Sargeras (in Medivh's body). Gul'dan just did as he was told.
I don't think it's fair that you can only choose between classes and not specs.
Even though I like the necromancer concept (Death Knights) a lot more than summoning Demons and stuff, I have to give it to warlocks.
Death Knights are kinda badass, sure, but they didn't necessarily choose their state. Warlocks did, simply to accumulate more power.
That said, Shadow Priests have some really, really nasty spells, too.