Hm... Now that I think about it, the only Pokémon I really liked from Kalos were Amaura and Aurorus. Kalos itself had many of my recurring favorites and gave them buffs, like Mega Gardevoir.
I like Kalos' dex a lot: it may be short, but it introduced many unique concepts. I personally like Chesnaught, Goodra and Aegislash's evolutionary lines the most. (As much as I love Goodra's line, I can't think of them without remembering how dissapointing they are in battle, tho. That and Chesnaught's Bulletproof not blocking Moonblast )
Sylveon, that's all I need, it's my perfect baby. No one hurt it, it needs to be loved. There are other Pokémon I like too, but Sylveon. It's the best. Also, a dragon murderer, how could we not like that!
AEGISLASH. I love the design and backstory, it's just an awesome Pokémon! (And the Shiny is one of the coolest shinies, in my opinion!)
I don't really like the Kalos Pokémon all that much (except for Sylveon), but a few weeks ago I came up with a story idea for Meowstic and fell in love with the Pokémon. Its now my favorite (accept Sylveon but that's a whole different category).
Mine would probably be a tie between the froakie line and the noibat line. They are some of my favorite designs and I honestly like the concept of a ninja frog and a dragon bat.
Mine is the entire Honedge line! Incredibly creative design, useful both in-game and competitively, interesting lore behind the concept too!
Espurr for somehow being creepy and cute at the same time! (Both Meowstic forms are great too) Sylveon is difficult to evolve, but has a great design and is a blast to use. Klefki is amazing for setting up status moves and has some amusing Pokédex entries.
Noivern, Talonflame, all of the starter lines, Diggersby tho, Slurpuff, Dragalge, Heliolisk, Tyrantrum, Klefki, Zygarde, Vivillon, I love all of them... It's so hard to choose! Kalos is so densely packed with many awesome 'mons. I guess Talonflame would probably be my favorite overall, if I had to choose.
Scrafty started off as one of my favorite designs and I was not disappointed from using it in battle. Honorable mentions: - Sandile line - Helioptile and Heliolisk - Tyrantrum - And oddly enough Swoobat has become a favorite since I raised one as a Calm Mind sweeper.
Phantump is looking at all of you like, "Oh so I'm not your favorite?" Definitely Phantump! I think the design is great and I really like Trevenant as well! I think the shiny form also another huge selling point for me! I hate when shinies aren't very different and the original is preferred... this definitely is not the case for Phantump!
Kalos doesn't really have any huge standouts, honestly. It's mostly forgettable stuff. If I were to choose a favorite, though, it'd have to be Greninja, with Heliolisk being second even if it's not close.