Iron Man. For a very good reason. It's simple, yet difficult. Iron Man has a deadly simple rule set. That's why I completely ignored it when it came out. "One ball game". But then I picked up a VE after they were out for a bit, and I considered it cheap. So I bought it on a whim figuring I'd get most of my money back out 3 months later when I sold it. And I was just blown away by the game. It's extraordinary.
I own a bunch of games, so a game has to have something special to latch into my collection. Sometimes they are here for a short time, and once in a while one just hits. IM was worthy of calling a keeper. I call it simple because the rules are so easy to understand. But I keep mine set up just a speck outside my skill level. That way I get that classic pinball perfection of easy to understand, but difficult to master. On the very rare occasion I get Do Or Die Hurryup, I feel like a pinball God. But then I'll play 50 games and not get close.
Oh, and fast games. 10 minutes will give me 3 games played. Good stuff, because if I want longer games, I move to a different title.