Batman/Predator was fucking awesome.
Batman/Spider-man is a personal fave. Both characters were treated well, and clear differences were shown between Joker and Carnage, when it came to their particular brands of lunacy.
Batman/Daredevil does for the heroes what Batman/Spider-man dud for the villains, playing off the differences in the characters, not just going on surface similarities. Crossovers that care enough to go beyond "both do the same thing so they're the same" are the best ones.
Both the above crossovers also have kick ass art which always helps.