Always the default 'normal' level. I then do harder levels if I enjoy the game enough (which is rare, I usually prefer to just move on to the next game) and if I'm really into it and there are a lot of trophies to get, I'll do what I can on easy later on (eg. for Uncharted 2, I got most of my "kill-so-many-people-with-each-gun" trophies on easy and a lot of them using unlockable 'cheat modes' such as one-hit-kills and unlimited ammunition).