Memorable: Pidgeotto. Appeared in numerous episdoes, even though it was never used in battle as much as it should've been. Ash's battle with Falkner and the part where he remembered his own Pidgeot has always been a special class moment for me. Ash and Pidgeotto were closer than any of his other bird pokemon.
Useful: Swellow, if only for its amazing ability to take hit after hit. It has a lot of key victories under its belt. Its able to take electric attacks and still get back up for more.