I don't have just one favorite... I'll just list my top 5 to make it easier on myself.
1. Gardevoir -- the very first time I played Pokemon Sapphire, I remember seeing Wally catch a Ralts and then saying "HEY I WANT ONE OF THOSE" and searching for hours to find one. I didn't know if it would be any good or not, it was a new Pokemon!! and I wanted it. Lo and behold, the thing's fantastic. Really helped in holding my team together, and ever since then, I've used one in every game it's available in. When Gen VI rolled around with the Fairy type and Mega Evolution, it only made it even better. (this was so goddamn spergy i'm so sorry)
2. Mawile -- Didn't use one until XY because I never had the right game for it, but I always liked its design. Again, the Fairy typing and Mega Evolution solidified it for me.
3. Mimikyu -- it's adorable and its whole thing about wanting to be as popular as Pikachu made me feel so sorry for it, so I used it in Moon and it's great.
4. Sylveon -- it honestly really surprised me. I loved using it in my team; it's actually pretty solid.
5. Alolan Ninetales -- my two favorite types merged into one Pokemon. Plus it has a gorgeous design. What more could I ask for?
(whoops, sorry for sperging there, i'm bracing myself for incoming :autism: ratings)