Hardest. In some games, like the Thief series, this implies a different way of playing too.

I always play the second highest difficulty. If I enjoyed playing on normal, I'll play through hard.

I normally play normal/medium first. If I enjoy the game, I'll replay on hard.
But for achievements/trophies, I will play through hard. Then go through normal for the achievements/trophies I missed.

I don't change the difficulty setting from what it is usually.

Unless it is like Pokemon Fighting Battle Simulator then I put it on easy because Charizard is such a beast.

two days ago I started metal gear solid 4 again but this time on Naked Normal

ME 2

Im currently trying to finish up my Mentor Mode game on ninja gaiden 2. For those of you who have played it, you know how insane it is.

^^^ This.

Normal in most games, and if fitting, I pick Hard i my 2nd playthrough. I always play MGS on Hard and then Extreme though.

Hard.

Games seem to either be stupidly easy or painfully difficult so I always pick the hardest one first and adjust from there.

I've only started one game at a lower difficulty (God of War) and then I regretted it later when I found out I'd have to beat it again on the higher difficulty anyway. -_-