Without wanting to burst anyone's bubble, I'd not go all out saying Togekiss is there to purely wall the main offensive dragons, especially since many of them can, and will, regardless of Togekiss's presence, run Stone Edge. It's not 100% reliable, of course, but that doesn't mean by any stretch of the imagination that Kiss' can safely switch in, especially once your opponent realises that Kiss is your answer to dragons, and so starts to predict her switch in. Kiss also retains her weakness to Stealth Rock, relies on Paraflinch to stop herself being KO'd by the dragons using Stone Edge, while she can only 2HKO many of them in return with Dazzling Gleam, without significant SpA investment. Kiss isn't the precise topic here, but my point is that she's not really the "most improved" when, short of the fairy typing, many of her old issues haven't been compensated for. On top of that, she won't really overpower UU so there's no reason why she won't be allowed there, regardless of a revived viability in OU from back in 4th gen.
My vote for one of the most improved (although perhaps not the most) is Blastoise, by means of his mega; along with being an excellent spinner, he gained something in value in the fact that he checks Aegislash very well; if a new pokemon will be used, so will its checks! Aside from those specifics, he gained a great new ability and buffed SpA from his Mega form, which, by nature of the ability, came bundled with a much improved move pool; he can get perfect coverage now from his STAB combined with Aura Sphere, Dark Pulse and Dragon Pulse (I don't recall if he got any of those before, but he definitely didn't get them all); before, he was fairly limited to his STAB and Ice Beam to have any kind of offensive presence in UU, thus rendering him inferior to Suicune, Milotic and Swampert in all ways but Spinning, meaning if you took him for spinning, you was bringing a slightly sub-par Bulky-Water to the field. The improved defences that come with his mega also make him more viable as a bulky water, although he still lacks recovery outside of rest, and loses out on what he had before in the form of Rain Dish (as setting up rain is really as long as it's broad for Blastoise nowadays) and, if you're using the mega to take advantage of the improvements, leftovers. Despite that, a RestTalk set is viable; nothing stopping him from filling your spinner role at the same time, and water isn't a drastically unreliable STAB - your spinner hardly needs to be a massive offensive threat anyway, but he might be outclassed by the ones who are.
