Well, since Unova introduced more genuinely good pokemon than pretty much any other generation, it's hard to pick just one. Krookodile, Scrafty, Mienshao, Chandelure, Reuniclus, Hydreigon, Volcarona, Bouffalant, Excadrill, Galvantula, Escavalier, Jellicent, Golurk, Lilligant, Darmanitan... all great pokemon - good designs, good stats and good movepools.
But one personal favorite? I guess it would have to be Leavanny. It just has an air of nobility that really appeals to me. It looks like (and was obviously patterned after) a swordfighting
cavalier (well... that and a praying mantis, which I also like). Bug/Grass is a useful offensive dual typing, and since I know how and when to switch, its weaknesses don't bother me, and it's got good speed and terrific attack. Mostly though, I just like it, and have from the first time I used one in-game.