Best skins for each character are:
Tracer: I like the aviation one with the glasses the best, though Sporty isn't bad, and I think her look works with Punk.
Genji: Oni. His Nipon one is good too. But I really, really, really, really wish they'd color coordinate his streaks as he changes costumes. It drives me insane to use his clearly red sword, but it leaves a green after image.
Reaper: Pumpkinhead and Nevermore. His default costume also looks good in red or purple.
Sombra: Noche for now. Not a huge fan of most of hers.
Pharah: I like halloween one. I would like her Gundam ones the best, but I hate how it makes her face a robot too. It ruins any PotG I get when she takes off her helmet to reveal a robot head.
Soldier: He doesn't have a best costume. He doesn't have a best anything. He's so boring.
McCree: I am actually a fan of his default, or Ebony. I feel they got just the right color scheme and design for him. His riverman one is pretty cool too, but I don't like him cleanshaven and combed over. Just feels wrong.
Bastion: I like his grass one and steampunk ones, but really, I don't actually care. Sorry, B.
Hanzo: His white outfit or Ghoul if I want something that's a little more different.
Junkrat: As annoying as he is, he has so many good ones. I like the Jester ones the most, I think, but he looks good in most of them.
Mei: Her pink one. It feels more appropriate than her default one.
Torbjorn: eh...default. Red seems to be his color, but I'm not a fan of his pirate or biker ones. They feel too comical.
Widowmaker: Ooh, ooh, her Huntress one is BY FAR the best one she has. I love it, she looks like something out of steampunk. Great gun change too. This should have been her default costume, not...whatever she wears in her default.
D.Va: I like her white rabbit one. A touch of Alice in Wonderland fits her, while still keeping her iconic pink color.
Reinhardt: He has a ton of good ones. I like paragon, stonehardt, and Balderich.
Roadhog: Mako
Winston: His Horizon Suit. It looks better than his plain white one, but also it's appropriate to his character.
Zarya: Her white suit, dawn I think it's called. Not that we have much to choose from here.
Ana: Ghoul. Definitely ghoul. Dat mask.
Lucio: I like that opaque visor he gets in one of his purple selections.
Mercy: Lots to choose from. She looks great in her amber suit, witch, and Imp.
Symmetra: I like both regal and utopia and usually pick one of those. I'm not big into her legendaries.
Zenyatta: Ascendant. I'm for anything that makes him look holier, and white coloring with purple sash is a good look for him.