JOOI, why have both responses so far explicitly suggested games I might be able to beat MHF at? Anyone would think I had a personal vendetta against him instead of vaguely wishing I played something better than someone :-)

In that case the answer is easy - pick an obscure game, study it, pick someone at random from the street who knows nothing about the game and beat them. But you don't really want to do that - you want to beat someone who's can play the game - which is where MHF comes in, because (like me, but I'm not in Cambridge) he's very good at learning the rules and picking up the strategy for games. So for this game that noone's heard of that you want to win at, he'll be a good opponent.

Besides, in this case, it's because MHF is the person who taught me philosopher's football, and thus would be the logical person to challenge.