Dustforce happens to fill all the criteria. Multiple characters, level editor and multiplayer (haven't tried multiplayer though. It's mainly a sp game).
Easy to learn, but hard to master.
Spelunky is pretty cool, has a high skill cap and an online leaderboard. Multiple skins, but they have no difference in gameplay. No customization, but randomly generated seeds gives a high replayability.
Osu, ossuming you don't play it already.