I vote Glasswing. It's an ultra rare and seems to be the strongest of the Champions. Rainbow Sentinel may be the same colors, but it's only a super rare. I'm trying to feed my Glasswing up to 15 but it takes A LOT of food, so it's been very slow going. But, even at level 11 (and a half, lol) she's already quite formidable in the arena. I've been using her for just about every battle, except when it's another Champ, and she takes opponents down with ease. She looks good doing it, too.
Those wings are fabulous.
I feel you on the Metal thing. Mine does a decent job, but my Fire and Playful can't fight their way out of a paper bag. Months ago everyone was talking about how great they are in the arena, so I fed mine up- with very disappointing results. It's weird how some dragons are great fighters on some people's Isles, yet basically worthless on other Isles.
So far, I'm a little suspicious of the red and black in the Dark One- and he's got such a loooong evo time. I'd hate to dump all that food and time into him just to find out he's an average fighter, no better than a regular Champ. Waiting for more feedback on his battle s****s before evolving him. It might be that his ultra-rareness trumps the color weaknesses...but it might not. :/