"A fictional crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story" – Wikipedia.
Simpsons – Futurama
This is actually a great crossover. Nice drawn and fun to read. There are 2 volumes. Both are great.
DC vs Marvel crossovers
There are a lot of crossover between these two different universes. Some of them are good.
Superman – Predator
There are actually some more stupid crossovers with Predator like Batman vs Predator or Judge Dredd vs Predator.
Joker – Mask
What the hell? How could they possibly meet?
Punisher – Eminem
This is just fucked up. Eminem! He is a super villain or what? People die because they hear his awful music so he should be punished probably.
Archie – Punisher
OMG! Why not Mickey Mouse meets Punisher? What is he doing on a ball with a gun? Probably he now works in security or something.