Least favorite is easy - Watchog. It's this generation's Raticate - just powerful enough and with just the right (wrong) attacks to be a pain in the butt to battle, which you have to do all too much, but not powerful enough to actually be worth having on a team.
Favorite is tough though - I like a lot of the pokemon from this gen.
Cinccino particularly stands out. It's a cute design, and with either Technician or Skill Link, it's actually pretty fearsome in combat.
Leavanny's a good one too - probably the best Bug/Grass ever, and with a great design.
I really like Reuniclus too. It looks cool, and it hits like a truck. Well... a psychic truck... like... thing.
Sigilyph is an awesome design, and Magic Guard is an awesome ability.
Scraggy is great. Scrafty's the better fighter, of course, but I love Scraggy's design.
Mienshao is everything I always dreamed about for a fast, fragile fighter, and its movepool is perfect.
Chandelure of course. I saw someone else somewhere say that it's like carrying a nuclear bomb around with you, and that's not far off the mark.
Excadrill is a monster. I liked Drilbur and kept one in my party because it was cute, but I sort of expected it to end up like this gen's Diglett/Dugtrio, and boy was I wrong.
Frillish is a great design and a great typing. I was a bit disappointed in Jellicent's design though. It's okay, but I really like Frillish better.
Thinking of that - I think Petilil is one of the cutest pokemon ever, full stop. And Lilligant is actually surprisingly dangerous.
Golett and Golurk are awesome in all possible ways - design, abilities, movesets, typing, everything.
And Braviary and Haxorus and Hydreigon and Stoutland and Beheeyem and Krookodile and Cohagrigus and Bisharp and Sawsbuck and Scolipede and.... pretty much everything but Watchog...