Zapdos   

RSE & LGFR RBY

ZAPDOS Slot 1 Slot 2 Slot 3 Slot 4 Thunderbolt Drill Peck ReflectLight Screen Thunder Wave Move Effect Type Pwr Acc Thunderbolt Has a ~10% chance to paralyze the target. Electric 95 100% Drill Peck Damages the target. Flying 80 100% Reflect The user receives 1/2 damage from Physical attacks until it switches out or either Pokemon uses Haze. Psychic N/A N/A Light Screen The user receives 1/2 damage from Special attacks until it switches out or either Pokemon uses Haze. Psychic N/A N/A Thunder Wave Paralyzes the target. Electric N/A 100%           Other Options: Agility, Mimic. Verdict: Zapdos is easily the best of the Legendary Birds in RBY because it gets a secondary STAB moves to work with. I don't know why Nintendo determined that only Zapdos should be capable of learning Drill Peck, especially since the movepools of the other Legendary Birds are sorta lacking, but dem's da breaks. Thunder Wave and its STAB moves are no-brainers, but the Screen of your choosing should be based on whether you want to boost Zapdos' Defense (for Rock/Normal attacks) or "Special Defense" (for Ice attacks)... considering this is RBY, you're probably better off with the later.