I haven't seen many but the ones that I've seen I've liked a lot. My favorite is the Gintama/Sket Dance crossovers. Not only were the jokes in both hilarious, but you could clearly see the differences in the style of comedy and drawing. I also like Cross Epoch, which is a crossover between One Piece and Dragon Ball and Carnival Phantasm (a crossover with the people of type moon). Oh! I think crossover videogames are also cool, I love the idea of Jojo's All Star Battle, which I've never played but having all the Jojo people in the same place is all kinds of awesome, and the Namco x Capcom series. I learned about a lot of really old video games while playing part of it.