Zapdos is the clear-cut favorite for me, and my favorite Pokemon until Gen 2 came along. What a great paradox that a bird could be Electric! Not to mention that Zap was one of the best competitive Pokemon in Gen 1 that wasn't Mewtwo. No double weaknesses, and one that can actually use its balanced stats as a result. I love the design, movepool and overall usage of Zapdos.
Zapdos was my Flyer and the only Legendary I caught in Blue when I was 8. Looking back, the Red/Blue sprite for Zapdos was really ugly looking! It looks so much better in the newer games.
I had a Lv.100 Zapdos in Blue and two Lv.100 Zapdos from FireRed and XD: Gale of Darkness. I have won many Battle Frontier medals in Emerald using Zapdos as my main.
Articuno and Moltres are great designs themselves, but they cannot compare to Zapdos.