hmmm.... Probably either Voltron or Transformers... Transformers first 'got me into it' but Voltron got me hooked. ASSEMBLE LEGS AND ARMS! ASSEMBLE TORSO!!! And I'll form the head!
Edit:
Oh poo!!! How could I have forgotten about robotech(/macross)!!! Its definitely between those three. The Holy Trinity of Mecha: Transformers, Voltron, Robotech.