Either Castle or Bones

Reid vs. Jane. :D

So HARD to choose!
I would love if it was with all of these but I don't watch NCIS

Criminal Minds. Castle a very close second though.

Any of them but I would love to see Jane vs. Reid like robothor11 said.

While I know this series is long over, I always wanted to see Red John and Jack of all trades go head to head. It would be awesome!

Edit: The only other crossover I would want to see would be Criminal Minds. While I like Castle and NCIS I don't think the mash-up would be a good fit.

or bones

Either this or Ghost Whisperer would be a pretty interesting crossover in my opinion.

it has 2 be castle. two hassled beauties with pain in the ass but lovable consultants. there can b so many subplots 2 - jane n castle brainstorming, castle can base a character on jane, jane n beckett both with their personal quest 4 vengeance, beckett n lisbon sharing their woes, castle flirting with lisbon...

Bones of course!