Here are some of my favorites in Gen 6:
1. Aegislash: It is my favorite overall because of it's typing (Steel/Ghost), the design of the sword and shield, and this pokemon helped me win the Elite 4 and especially the Champion almost single handedly (running King's Shield, Iron Head, Sacred Sword, and Shadow Claw). Also, Stance Change is a wonderful ability that gives Aegislash some versatility compared to other gen 6 pokemon.
2. Aurorus: I'm a huge Ice type lover and despite it's weaknesses, the design of the pokemon was superb and it has access to Freeze-Dry (which is great for pesky water types).
3. Chesnaught: This was the first fully-evolved starter in Gen 6 and I enjoyed ever second with Chesnaught helping me win my battles against Team Flare.
4. Greninja: Great typing, excellent movepool + protean makes Greninja a winner.
5. Hawlucha: I've been waiting for a Fighting-Flying type for a long time and Hawlucha fulfills my desire and then some.