Ironman/Harry Potter? Thats a pretty strange crossover, especially since they had to explain why every known magic user in the dc world ISNT a part of the "magical world" in harry potter. But watching harry attack the troll in first year with his own tech suit was fun. too bad it didnt work very well.

BtVS/Superman. BtVS/Magic the Gathering. One is just silly for how instantly overpowered a kryptonian is against vampires, the other was just insane, Slayer Magic on fanfiction.net. Both are those halloween fics that justifies instantly any crossover you can think of.