I really like both Chandelure and Golurk, but I have to give it to Chandelure for this one. That thing is the definition of powerful, with the highest Special Attack of any not legendary/banned/Mega Evolution in the series (only behind Aegislash for legends and Megas). Fire Blast or Overheat, and it doesn't care what it comes up against, if it doesn't resist Fire, it's going down. It is disappointing that Shadow Tag was never released, but its sheer power makes up for it.
Golurk I just talked about in the Favorite Ground Pokémon thread in Pokémon Polls, but i'll mention it here anyway - very high Attack, above average Defenses and some nice moves to use them with makes for a pretty cool Pokémon. Its design is very reminiscent of Transformers, which is cool. It's kind of a shame that it comes so late in the game though (especially in B2W2), but XY remedied that with being able to catch it before your second badge. And it can fly, which is nice.
I have to give a mention to Jellicent, too, if only for the mustache that the males rock.