I've got my marks hotkeyed on my numpad, so I just really tab through my enemies and press my numpad keys to mark 1 - 5. I don't understand why most tanks I've seen in DF don't mark, as it really makes your life, and that of the DPS and healer a lot easier. (provided people follow the marks, of course)

Anyway, my marking order depends on whether or not we have someone who can sleep enemies in our group. If we do, unsleepable mobs get kill priorty. If not, it's healers > casters > melee.