eventually you will play as the only legendary skin you get.
Not counting the ones that came with the physical edition, I've unlocked four legendary outfits. I'm only level 17, so luck has been kind.
I've got Hanzo's Lone Wolf, which I think is one of the best looking in game. I'm not that great playing him yet, but slowly improving. It meets your definition in that a character I might normally skip over will be a top five played now. Just a fabulous skin.
Mystery Man for McCree, but can't stand the character. He feels incredibly slow in a game that rewards reaction and some twitch play. I see people do well on him and am fairly astounded. He's basically ultimate or die for me.
Stonehardt for Reinhardt, who I really dig. Probably my favorite of his legendary outfits, right next to Blackhardt. He'll likely be my second most played, since he's great in a good team.
But my pride and joy is Mercy's Sigrun. Absolutely love to the character and love the outfit. It would be the one I would purchase if given the option, so I'm stoked. It was my first unlock. Guess Blizz blessed me for all those healing hours I've logged. Mercy is the kind of character that can turn a good team into a great one.