I usually start on normal for Single player to enjoy it, unless like someone else said, reviews say it's stupidly easy (Enslaved for ex.). Games I like and that give achievements I'll go through again on higher. Mass Effect for example, I went through on every difficulty on xbox. Insanity wasn't hard with a maxed lvl 60, just takes a lot to kill the baddies, especially with the Immunity spamming enemies.
