1. Superman vs Spiderman
2. Batman vs The Incredible Hulk
3. Superman vs Predator
4. Batman vs Aliens
5. Superman vs Aliens
6. Batman vs Predator
7. Superman and Batman vs Aliens and Predator
8. Green Lantern vs Aliens
9. Judge Dredd vs Aliens
10. Predator vs Judge Dredd
11. Batman vs Judge Dredd
12. Batman vs Dracula
13. Apocalypse vs Dracula
14. Avengers vs Xmen
15. G.I.Joe vs Transformers
16. Godzilla vs The Avengers
17. The Incredible Hulk vs Superman
18. Superman vs Terminator
19. RoboCop vs Terminator
20. Aliens vs Predator vs The Terminator
Basically Batman, Superman, the Terminator, the Aliens and the Predators are constantly at each others throats.
But then, who wouldn't want to see that movie!