In my experience, OpenFeint was the easiest to implement into my game. It only took a few minutes to have the code up and running and all the menus showing up. I think it's also the nicest looking one(other than plus+). It's free and has tons of users because it's in Pocket God and some other big games.
Agon is also totally free, and has really nice developers who seem genuinely interested in helping to make your game better, it just didn't seem to work right in my game.
Scoreloop is interesting, but their whole model is based around coins and challenges, and that doesn't really matter in my game. I didn't want to make users think I was asking them for money or something.
Plus+ is cool, but you've gotta sign an NDA with them, and let them approve your game, and probably do some sort of publishing agreement(or at least some profit sharing) - it's really cool, but not worth it.
If you still wanna do more research, check some of these links out: