Depends on the game. Usually normal... or even easy.

That way I can go back and play harder versions and get further enjoyment from it rather then it just feeling like it's too repetitive when i want to play it again.

Most of the times I would start on Normal.

If I'm new to the game, I pick the normal difficulty. After I beat it, I play easy, then hard.
If I know the series already, if I feel like it normally is too easy I start on hard. If I'm fine the way it is, I do either easy or normal, depending on if I want to get through the game quickly or actually want the challenge.

if i know there ill play and finish the game more times, i choose easy first. then medium then hard, usualy
when i im not sure how many times ill play, i chose medium.

hardest one unlocked.

I always play on the easiest level. When I was a kid,it was always on the normal mode. I'm old and most of the time I just play the game for the experience and not the challenge. It was the exact opposite when I was a kid.

Normal.. except a few times when i was a class A trophy Whore back in 2010 playing some games on hard from the get go to get the platinum trophy faster.

now i only play on Normal first and foremost.

I start on Normal. I don't play games on higher difficulties unless I like them enough to warrant an extra playthrough. If I don't like a game enough to do this, it takes more time and effort than I'm willing to put in to complete the game on the hardest difficulty. A game must prove to me that it's worthy of my effort.

Always the hardest difficulty, if the game is so cheap and boring then I stop playing it, if the game actually is enjoyable and its genuinely difficult then I'll continue playing it till I beat it.

I really hate unlockable difficulties, if you want me to play your game again then add something to it, not a harder mode.