Choice Band Gyarados: It's nice for breaking but I think it sacrifices part of what's good about offensive Gyarados
Choice Scarf Gyarados: Same as above really except exchange breaking for RKing/cleaning
RestTalk Gyarados: This is actually pretty cool for getting up multiple boosts as Mega Gyarados, but I think the key thing holding it back is that you only need to be able to 3HKO it and that it is a bit reliant on getting good rolls while asleep in order to function effectively.
Taunt Gyarados: A commonly used set on Mega Gyarados a while back for it's ability to stomp on stall (and for what it's worth this should be listed as Taunt Mega Gyarados not Taunt Gyarados). It allows it to dodge burns from MSableye through Magic Bounce thanks to Mold Breaker, it can completely shut down Whirlwind users (Circle Throw/Dragon Tail is a bitch but users of them aren't dealing with Gyarados well anyway unless ur talking about Poliwrath) and it can dodge Thunder Waves from 'mons like Chansey--among other cool things.
Natural Gift Gyarados: Tried and tested lure. Really nice for breaking through common checks to offensive DD Gyarados. Note that another option is Cheri Berry, which sacrifices a KO (I can't remember which) for the ability to dodge a T-wave if you don't need to use Natural Gift in a match.
Bounce Mega Gyarados: Another tried-and-tested lure, this is nice for it's ability to put both mega and non-mega Gyarados to good use beyond setting up and dealing with EQ/fighting users. Not being cockblocked by defensive Grass-types is rlly nice in general.
Rocky Helmet Gyarados: While I understand the logic here, I think that Helmet sacrifices too much longevity to be worth it compared to Leftovers. The passive recovery is really nice to repeatedly check things and to edge out of KO range for stuff to ensure you can Rest up again.
Choice Specs Gyarados: Lol
It's a really tough call 'cause I wanna vote for both Taunt and Bounce, so I flipped a coin and Taunt Mega Gyarados won.
Edit: Changing vote to Bounce Mega Gyarados 'cause I've thought about it more and I like the concept enough for it to edge out Taunt for my vote.
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Also why is Choice Specs winning this? Like I get that it's a funny meme and that it is fun to meme a bit, but when these memes actually win it is going against the entire purpose of the project (to vote for the best non-standard set). When things like Choice Specs Gyarados and fucking Skull Bash Mega Venusaur are winning weeks it is just plain stupid.