Hardest possible, unless it's maybe God of War which I play on Hard. Normal difficulty in all games is a joke, a meaningless waste of time.
Don't be too humble.
OT: Start out with normal, normally. If it's a notoriously hard game, I might go easy. Of course I replay games at higher difficulties for fun/trophy purposes.
But for people that say they play EVERY game at it's highest difficulty are simply lying, or have no life.