Zapdos is my favorite of the three birds. I've always liked its typing, its appearance, and its speed. I used both Zapdos and Articuno in Red (well, more recently; I only used Blastoise at first), and they've both served me well. Zapdos fills in the gaps better, though, as my Blastoise already knows Blizzard, and Zapdos has Drill Peck. I've never bothered with Moltres; it always seemed weaker to me in gen 1, and there have always been other fire types I prefer.