Even though my favorite type is Ice, my favorite Pokemon is Ho-Oh. Nowadays I don't use legendaries, but back before I got into competitive play, Ho-Oh was my main Pokemon. It originally became my favorite Pokemon because I used it to beat my cousin who has awesome video game skills who I had never beaten before (at any game). He was using legendaries as well (like Mewtwo). However, when I later got into mythology, the phoenix became my favorite mythological creature which added to Ho-Oh (Houou, or Fenghuang, aka Chinese Phoenix) being my favorite.
Here's a rough list of my top 10 favorites: Ho-Oh, Mew, Wartortle, Meowth, Dunsparce, Froslass, Vaporeon, Leafeon, Lickitung, Sneasel
Favorite Pokemon of each type (no repeats): Shedinja (Bug), Sneasel (Dark), Dragonair (Dragon), Raikou (Electric), Lucario (Fighting), Ho-Oh (Fire), Articuno (Flying), Froslass (Ghost), Leafeon (Grass), Sandshrew (Ground), Lapras (Ice), Meowth (Normal), Nidoran Male (Poison), Mew (Psychic), Shuckle (Rock), Skarmory (Steel), Wartortle (Water)
Of course, some of this will probably change when all the Gen. 5 Pokemon are revealed.