The Fury VS the JLA
Alright, this should be a good one The freaky adaptive cyborg that killed all the heroes on an alternate Marvel Earth gets shunted to DC Earth. It's programming is to kill any metahuman on whatever planet it happens to be on, and the current JLA lineup is the first line of defense. For reference:
http://www.marvunapp.com/Appendix/fury1.htm
http://www.internationalhero.co.uk/f/fury.htm
Frankly, I think the Justice League is boned.