who's the better nemesis?  

This is assuming that we're only talking about the Klingons & Romulans. Actually, the borg was the most formidable, as was that Speciies 8472, or even The Dominion.

It seems that somewhere in between the original Star Trek & The Next Generation, the like it ws said by Ayleborne in TOS: Errand of Mercy, The Klingons & Federation did indeed become fast friends. The Romulans didn't until The Dominion War, & it was probably only temporary, being that there were lots of disagreements. I therefore feel that the Romulans were the better nemesis.

oh definately the klingons - the romulans are just vulcans with an attitude problem, not creative enough in character to keep my interest as well as the klingons.

I guess Romulans...

Romulans showed a lot of hidden technology in star trek nemesis as well as filthiness...they cannot be trusted by teh Federation even after they became a temporary ally for in teh Dominion war...

Klingons are a bit more trustworthy...

I think Klingons are more awesome, but Romulans are the better enemy. You see them throughout Star Trek as a tricky, deceptive race. "It's always a game of chess with them," Picard says. The Romulans have too many tricks; you don't feel safe with them. The Klingons are very honorable. They would never use any dirty tricks against the Federation. The romulans would.

the romulans more cunning,have better technology and unlike the klingons have no honour shown in there attack on the klingon outpost worf was on as a child and the planned surprise attack on ds9 so they could destroy the wormhole this was averted by chief o brien who was time shifting into the future