I think easiest is some sort of breakout / pong-like game.
Then maybe a top-down shooter or sidescrolling shooter? If you don't raise your bar too high, a platform game should be possible then too..
You should definitely do some of the tutorials
here. I don't want to advertise my own tutorial on that list (Bermuda Triangle Tutorial) but (oh dang, too late :P) I think it can get you started making a game and getting an idea about how stuff works in MMF. I also recommend the 2 Player Pong tutorial, best before attempting one that is more tricky.
Ooh. It would be nice if there was a note how hard each tutorial is on that list. Hope Jeff is reading this