Star Trek

I would say the Borg, it would have been awsome to see Kirk and crew up against the Borg

I am curious what the Borg would have looked like in the 60's/70's. I think Kirk and crew vs the Remans from Nemesis would be pretty cool, as well as the Borg, and the Dominion. 

In a similar vein, I am curious to see how Sisko would handle a promenade full of space hippies, or any of the TNG era captains (Picard, Sisko, Janeway) vs the Suliban, who I believe we've only seen in Enterprise.

I think TOS vs the Borg wouldn't be so great. I doubt 23rd century technology could even make a mark on a borg cube.

I'd like to see them up against the Dominion.  It would be interesting to see if  dominion ships were any different back then.  It seems like Starfleet is always an equal match against the villians (exept the borg) but I bet the Dominion was stronger then. Didn't Weyoun say the Dominion was around for a thousand years?

He did indeed. And I imagine the old Enterprise may be able to combat borg from the 23rd century, but not present day borg.

I love my TOS, but I don't think they had the tech to fight the borg back then...So personally, I think I would like to see them up against the Cardassians, because they used a lot of mind tricks, (i.e., THERE ARE FOUR LIGHTS!) and the TOS crew worked best fighting mental battles.