For the sake of all that is holy, please do not give any OU mons or mons that are already great in rejuv a crest. Crests are meant to take mons that are looked down upon into the late game, not to make your favorite Pokemon ubers material.
Anyways, while I'm here.
Muk Crest: Swaps Defense with Special Defense, and makes any contact move used on it, apply Badly Poisoned status to the user of the contact move.
Reasoning: "It is usually undetectable because it blends in with the ground. Touching it can cause terrible poisoning."- FireRed Dex Entry.
You could change badly poisoned, to recoil against the user of the physical move too, and it would still make sense. You could also use that dex entry to justify it having a ground type motor drive of sorts, although that's skuntank's thing. An alternate cool idea would be to swap it's attack with it's special attack and a "Water Bubble" ability for poison. It is literally a giant glob of poisonous sludge. It's more then justified.
Electrode Crest: Swaps Special Attack with Speed. If it gets hit with an attack before it moves, the next electric attack it uses would be twice as powerful .
Reasoning: "SoulSilver: It stores an overflowing amount of electric energy inside its body. Even a small shock makes it explode." "X: It explodes in response to even minor stimuli. It is feared, with the nickname of “The Bomb Ball.”"
Offers an alternative way of play. I think this makes the most sense for electrode, since outside a stat boost and maybe a free magnet rise, there isn't much else that fits, bar maybe an Electric type water bubble/Electric Surge, which aren't going to happen with 130 speed and decent spA for obvious reasons.
And finally, the mon who deserves a crest most
Greninja Crest: Applies Battle Bond and Water Bubble.
Greninja is a very underused and garbage Pokemon. It's movepool is pretty mediocre and it's special attack and speed are dismal. I mean come on, getting outsped by Jolteon? Pitiful really. I think this crest would make it at least RU tier in Smogon, although given how bad the Pokemon is currently, there's no telling what would happen. But it's one of my favorites so I think you should give it a chance.