For me, it is definitely Lapras. It's a Pokémon that i've loved since Generation I for not only its gorgeous design, but also the fact that it gets so many cool moves for a Pokémon meant to transport across water. I've used it in so many playthroughs of RBY, GSC and their Gen III and IV remakes, and my main playthrough of X. Even if it's not that good competitively, I still love it. But the sad thing is, even though BW2 are my favorites in the series (ugh, I always juggle between them and XY), I have yet to use one there ever, mostly because of how troublesome finding it is and how late in the game you can get it.
My other two are Houndoom and Scolipede. Houndoom because its design and attacks have always been really neat, and it getting a Mega Evolution in XY was even cooler. And Scolipede simply for its coolness factor. I mean, where else do you see a BIG FREAKING BUG that not only is very fast, but can use Aqua Tail, Superpower, Earthquake and Rock Slide? And getting Speed Boost in XY makes it even more f***ing boss. So yeah, those two are ones i'll always love as well.