Poker: The AI is really hard.

Blackjack: The AI on this is really dull; you just implement a set of rules you can look up anywhere. I’d rather you do something that you need to think harder about.

Bowling / football / other sports simulations: These usually turn out really bizarre. (I have seen a successful basketball free-throw game, though. - JLND)

Some kind of adventure / role playing game: This nearly always devolves into a massive set of nested if statements that don’t really give you the opportunity to think about more interesting algorithms. This can be done right, but it’s easy to miss the mark.

Risk: Getting the map is pretty hard.

Settlers of Catan: The rules are complex and representing a hexagonal grid is not easy.