While i'm not sure if its the most improved, i think Crawdaunt at least deserves a mention here since it got buffed quite significantly this gen.
First off, Crawdaunt finally got one of the things it needed most: a decent priority move. There's no doubt that Crawdaunt hits pretty hard, but pre x/y it was notable that Crawdaunt was still somewhat slow after a dragon dance, making it sorta mediocre. Now that Crawdaunt has a powerful Aqua Jet to work with (slightly stronger than Azumarill's Aqua Jet and even Diggersby's Quick Attack in fact!), it doesn't have to care about how fast it is, as it can simply batter faster, frailer opponents with an Adaptability boosted Aqua Jet, while running enough Speed to outpace and smash frailer opponents with its stronger STAB moves, speaking of stronger stab moves....
Crawdaunt also recieved two great STAB moves this gen in the form of a newly buffed Crabhammer and Knock Off. Crabhammer was often seen as worse than Waterfall last generation despite having slightly higher BP because of its lower accuracy. but x/y has given Crabhammer 10 more BP, making it an 100 BP physical water-type move, a significant step up from Waterfall despite the slightly lower accuracy. Combine this new base power with Adaptability and Crawdaunt has a massively powerful 200 Base Power move to spam, which is strong enough to OHKO even most reasonably bulky Pokemon and 2HKO decently bulky resists such as Azumarill with no prior damage.
252+ Atk Choice Band Adaptability Crawdaunt Crabhammer vs. 252 HP / 0 Def Azumarill: 204-241 (50.49 - 59.65%) -- guaranteed 2HKO after Stealth Rock
Knock Off got a pretty cool buff this gen too, its still only 65 base power, which is weaker than Crunch and even Night Slash, but it still has the benefit of removing the opponent's item and it happens to get a 1.5 power boost when it does this, making it only slightly weaker than Crabhammer on first hit! Its pretty obvious no Pokemon wants to be on the recieving end of that Knock Off, even if it gets weaker afterwards.
Lastly, while indirect, the Steel-type nerf helps Crawdaunt out quite a bit too, as it means that various Steel-types such as Ferrothorn will no longer be able to switch into its Dark-type moves with impunity, in fact taking surprisingly high amounts of damage if they try and do so (Knock Off does like 65-77% to standard ferro on the first hit, holy fuck lol). This of course makes Crawdaunt's Dark-type moves much more spammable and decreases the need for moves such as Superpower, which is always nice.
So all in all, while i still don't think it'll break the OU mark, i think Crawdaunt got buffed pretty significantly this gen, and i could totally see both Swords Dance and Choice Band Crawdaunt being significant threats in the x/y lower tiers, whichever one it ends up in, what do you all think?
