My favorite always has been and always will be Alakazam. I've never been a much of a Pokemon player, almost zero competitive play and I've only completed a couple of the video games (as a kid I got fed up with the games because I couldn't just level up an Abra to get Alakazam, had to do the stupid trading thing and nobody I knew had the cord for it), so my pick is actually based largely on the card game, which I was into when I was a little kid. Everyone else was all about Charizard and stuff, but once I read his description thing that said his IQ was estimated at 5000 I decided he was infinitely superior to silly dragons. I ended up getting 3 different versions of Alakazam, all of them holographic cards (which was the coolest shit when I was a kid, sort of equivalent to shiny versions in the video games), by trading with friends. The best part about it was being better at the game than said friends and smashing their conventionally cool Pokemon with my ridiculous Alakazam deck.

So yeah, Alakazam is my sole favorite both due to nostalgia and really liking his design/concept.