Hmm...it's a hard decision, since I always use the main birds in all of the generations.
Though, if I had to pick, it would probably be the Pidgey > Pidgeotto > Pidgeot line. One, I love it's design. Two, Pidgeot is pretty fast and its helped me out of some tough situations. Three, I've always used Pidgeot, both in Gen. I and Gen. II, so I've really grown attached to it.