I can't say there's one that's the outright worst, but there's quite a few offenders out there:
Greninja: Edgelord frog is not a good combo, and when you take the ninja characteristics into account, you now have an edgelord weeaboo frog, which is an even worse combination. Not to mention its hideous design (the fake 'Gallantoad' design was miles better), it really is the worst of the Gen VI final stages.
Delphox: I'm sure you've heard this opinion a thousand times, but Delphox was a massive step down from Braixen. Let's go from an interesting mage-fox, to a fox with bell-bottom styled fur on its legs and ear hair thicker than the rest of its fur. Not exactly a great design, especially considering the potential of Braixen.
Pidgeot (not Mega Pidgeot, mind you): Can it get any more generic? It doesn't have any notable qualities. Compare it to the other early route birds. Noctowl was notable for being, well, a wise owl. Swellow had a sort of swagger to it, with its sleek design and puffed up chest, giving it a very interesting 'bully' theme. Staraptor is, in my opinion, the golden standard of an early route bird, fast, hard hitting, kitted out with an excellent physical movepool and an intimidating (heh) cry. Unfezant, while it lacks in actual gameplay potential, has the most distinctive design (along with the only proper example of sexual dimorphism), with the flamboyant male and understated female. Talonflame is relatively simple, GF decided they had to one-up Staraptor, so they lit it on fire and gave it Gale Wings. Meanwhile, Pidgeot is a really big bird, which, unfortunately, is something that other species have done much better.
Miltank: Thanks, Whitney.
Jynx: Yes, it may be purple, now, but it still looks like a badly executed racial caricature, which doesn't reflect well upon the franchise as a whole.
The entire Tepig line: I feel like the root of Tepig's name, 'tepid', describes this family well. Not only were they the third Fire/Fighting starters in a row, they were also the worst of the three, trading in the speed and power of Infernape and Blaziken in for a beer gut.
Barbaracle: I've seen some depictions of Binacle that manage to look alright, but I have yet to see Barbaracle as anything other than a malformed abomination that should have been vetoed before it left the concept art stage.
Dugtrio/Magneton: What's the difference between between a Dugtrio/Magneton and a trio of Diglett/Magnemite? A Dex number, and some Gorilla Glue. Weezing nearly falls into this category, spared only by that strange tumor-like approximation of a third head.
Muk: People may rag on Garbodor, but at least they made an effort to create an original design, rather than enlarging a copy of its preevolution.
Sylveon: Somewhere between Generations IV and VI, GF forgot that Eeveelutions are both cuddly and intimidating, and gave us something that really doesn't look like it was intended for battling at all.
Granbull: The concept is interesting, but it really looks dire, with the strange, spiky bottom jaw, leaving the rest of its body featureless and pink.
I think that's all for now, unless someone else reminds me of a species that really provokes my ire.
~SS
