That's really difficult....so many options! I personally think Soul Eater would be a great crossover with anything; I'd especially want to see it with Kuroshitsuji. But really, sitting here thinking, I can come up with seemingly endless possibilities. I don't know if I could choose just one!
bleach and fma would be weird but cool and fruits basket and ouran would be funny. what would happen if someone put death note with bleach?
pic is ouran and fruits basket link