My favorite used to be Metagross, I mean who doesn't like 4 legged supercomputers with psychic powers? Guy was the main reason I loved Steel types the most, at least that was before.
Nowadays I'd say it would be a toss up between Mimikyu and Volcarona, the former being an adorable and fascinating pokemon that cements itself in the competitive scene with a gimmick that works and the latter being a novel Bug-Type, and one of my favorites to use against others, in a Rain team no less. (plus Volca looks awesome!)