Even though I don't have White yet, I have a feeling Braviary will soon take its place as my all-time favorite.
Until I start playing it, though, I'd have to say Pidgeot. It's served me well through countless play-throughs. I like Noctowl, too, even though it's stats aren't all that great.
Edit: For the legendaries, I love Articuno and Moltres. Lugia's great too, and even though I didn't used to like Ho-Oh very much, it's grown on me since my last play-through of HeartGold.