I would just like to mention that design and lore is what I find most important about a pokémon. Battle capabilites aren't a concern of mine at all.
My favorite gen VI pokémon is probably Heliolisk. I think both the idea and execution of mixing a frilled lizard and a solar panel is is a brilliant idea. It clearly manages to stay away from the "real animal with lightning stripes" problem that quite some Electric pokémon suffer from. I also greatly enjoy the way it looks although it is a bit hard for me to explain exactly why. It stays true to its source material while still being an original idea. It neither looks too detailed nor does it look too goofy with too cartoony body proportions for my taste. I'm not a huge fan of GF slapping on Normal typing to it for no particularly good reason though.
My top 10 in no particular order would be: Fletchinder, Floette, Honedge, Doublade, Skrelp, Dragalge, Heliolisk, Phantump, Bergmite and Yveltal. In general I think Kalos introduced pretty neat pokémon.
e: decided to add some more details for the rest too.
Fletchinder: Having a regional bird not be Normal/Flying is interesting. I especially like its color scheme and how agile it looks. I wasn't that fond of Talonflame since the yellow doesn't stand out as much on it and I think the torso looks a bit too big compared to the rest of its body.
Floette: I think it is the perfect example of what a Fairy type should be thematically. Oddly enough I really like Floette's design while I couldn't care less for Flabébé's despite looking very similar(I don't care much for Florges either but the difference between them are more drastic). Floette is the only one that I think actually looks cute of them. Blue flower is my favorite, with Eternal being second. I kind of wish it would have been Fairy/Grass though.
Honedge: I love pokémon that have some extra details that you can't really portray in static artwork alone. In this case I'm referring to Honedge's unsheathed and sheathed forms. The unsheathed sword has a really slick design in my opinion, while the seathed form has great coplex Celtic design. I also like that the edges of the sword turn into a face when sheathed. Not to mentioned that a cursed sword is an awesome idea seen in many tales.
Doublade: Pretty much the same exact reasoning as Honedge. I don't think that evolutions which are just two-three versions of the previous form are necessary bad and it works great for Doublade. I also think it has some of the best animations, which is quite impressive given that it consists of two rigid swords. Aegislash is also great but isn't quite cool enough to make my list, mainly because the difference between the sheathed and unsheathed forms aren't as different despite actually being different formes game-wise now, and that the handle doesn't look like a handle anymore.
Skrelp: I think seahorses and sea dragons are great creatures, and I am happy we got a leafy sea dragon this time. Skrelp's frail and miserable figure is something that we don't see that often in pokémon(at the very least not outside Ghost types) and it makes it quite charismatic and a great parallel to Horsea.
Dragalge: A wonderful evolution to Skrelp. It strikes the perfect balance between looking frail and magnificent, like it has fully embraced the poison. Exactly how I think a Poison/Dragon pokémon should be. It is also still a great parallel to Kingdra which is a weedy sea dragon, both being pokémon I really like.
Phantump: I think it just looks damn cute. Its simple body works very well with the more detailed stump it has for a head. I also like that despite looking cute it has a more sinister backstory to it that also make me kind of sorry for it. I like Trevenant too but not enough for it to make the top 10.
Bergmite: This is a pokémon I like mostly because of the execution. Despite just being a chunk of ice on legs I think it looks really quirky. Avalugg is kind of the opposite. An iceberg hangar is a neat idea, but I felt that the execution could have been done better, I also miss the white parts from Bergmite to break up the ice coloring. I still like it though.
Yveltal: I liked it from the start. Life and death are themes that many have expected to appear in legendary pokémon and I feel both Xerneas and Yveltal delivered. They also weren't made into literal rulers of life and death but just being able to manipulate it to a great extent, which is something I am happy about. I was a bit afraid that they would make it into yet another legendary Dragon mascot but luckily they didn't and Dark/Flying fits it very well. It managed to avoid all the things I fear can happen when new legendaries are revealed, and for that I am thankful. Just like Honedge Yveltal has a dynamic design, in this case I'm referring to how its body light up gracefully when it uses Oblivion Wing, but also its dormant pupa state. I still don't really feel confident about what it is based on, but there are no parts of it that I feel need deeper explaining to make sense, and in general I think it is an awesome pokémon on its own. As you probably understand I also like Xerneas, and regular Zygarde(I know too little about it to make an opinion on the rest), but as I mentioned earlier I like a lot of gen VI pokémon and they didn't manage to make it to the top 10.
