In my first playthrough of FireRed I caught and used Moltres and Zapdos in my team. They were both helpful additions even if having two Fire/Flying types may have been a bit silly. Four flying types in one team is crazy but I couldn't not use a Pidgeot while playing a Kanto game, right?
However Articuno is my overall favourite of the three. I just think it's design is really elegant, and ice types have always interested me. I also found the fact that it could learn Sheer Cold to be bittersweet as it would never work for me but always would work for my friends against me!