I like Merlin/Gwen myself and not sure how I feel with Arthur/Gwen--I'm mixed on Arthur/Morgana (and you know me, I don't slash without like...proof that it's canon or that the character is actually not straight) and don't think there's really a character that goes with Arthur yet.

I think my main issue with Gwen and Arthur is they seemed to have changed Gwen so much from the original legend (not just personality, but looks as well), that her with Arthur seems kind of...off. And we all know she's going to end up with Lancelot anyway so I'd probably go with him or Merlin, but it just doesn't work with Arthur.