My recent favorites, Mismagius and Dragonair, are based on how elegant they look, along with their worth in competitive battling (Standard and NFE Battles, respectively).
Of course, there are some special mentions, like Latios, being the Pokémon that helped me conquer the Emerald Battle Frontier, Jolteon, being my first Eeveelution, and Blastose, being my very first starter, ever.