Il'gynoth lifesteal OTK Demon Hunter is quite playable at all levels, including high legend. It can deal potentially deal north of 100 damage on a single turn when the combo goes off.
Malygos Druid is an OTK deck at its core (although it has alternative win conditions too) and a pretty good one as well
Hunter only has some meme OTK decks that involve hand-buffing charge beasts
Mage has a (not so reliable and well-performing) OTK that involves playing Archmage Vargoth, followed by Potion of Illusion, followed by a 1/1 copy of Vargoth. This will give you 4 1/1 copies of Vargoth in hand and on the combo turn you play 3 of them followed by Mask of C'Thun to send 40 damage to the opponent. Mozaki Mage is another OTK option, but I think it is even worse than the Vargoth/Mask of C'Thun one.
Priest only has this Miracle C'Thun deck in terms of OTK, Shaman only the Quest-Corrupt one, Warrior only the ETC or the Silas, Ashtongue combo
Finally, I'm not aware of any Paladin, Rogue or Warlock OTK decks in standard
Besides what the post above said, only other OTK deck I am aware of is this Priest OTK involving Reckless Rockateer, Embalming Ritual and copying it with Grave Rune, then popping it with removal. Not sure how good it is though.
There's a shaky as flip DH OTK deck in standard with expendable performers, the naga with a health buff, and immolation aura. Shaky as flip because you NEED a Skull to hit a combo piece or you can't *actually* combo. Probably better to use Ilygnoth, though, by far.
I run an OTK deck that revolves around copying buffed versions of King Krush by using Ramkahen Wildtamer/Hunting Party then using Beastmaster Leoroxx to unleash them all at once. The version I use has lots of card draw and some secret options to cheat cards out of your deck quicker to get your combo pieces. I would say the OTK is usually turn 10 on average and I have successfully completed it by turn 8 before. Let me know if you want a decklist.
I've made this maly rogue, the full combo can deal 52 dmg or more if u manage to save preps and evis, but its flexible since its an 1 mana maly, u can kill ppl at turn 7 with godlike draws, see if u like it :)
There's a hilarious new take on the APM Priest of old in wild. It only takes 4 non-common wild cards and can go off turn 5 or 6 with a bit of luck and a very fast play thru.
A Youtuber who goes by Clark Hellscream put up this version
2 Circle of Healing, 1 Lazul's Scheme, 2 PW: Shield, 2 Regenerate, 2 Silence, 2 Topsy Turvy, 2 Holy Smite, 2 Wave of Apathy, 2 Insight, 2 Shadow Visions, 1 SW: Death, 2 Gift of Luminance, 2 Nazmani Bloodweaver, 2 Palm Reading, 2 Gadgetzan Auctioneer, 1 Psychic Scream, 1 Mecha'thun
Hilarious to watch that kill turn six against basically anything.
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