Having played almost every sports game (aside from golf or tennis), I would say NHL or NBA games would be the easiest. I recently got NHL 2011, because my friend recommended it and it's not challenging at all, and I'm definitely not a hockey fan. NBA, also is easy to grasp. Your players usually get the rebounds and defense is usually taken care of so it's not too challenging.
The hardest for me is definitely baseball. I watch it, but I suck at the game. In MLB 2K10, I could barely hit the ball, so I quit. Madden is easy for me, since I know a large amount about football and have played it a lot, but it shouldn't be too hard to grasp.
But I would say NHL, 'cause all you do is slide around a court and you can make plenty of shots and still suck. (in the game, of course)