Voltron - anybody notice how similar in appearance is Optimus the Transformer?!
Voltron came first.
Mazinger Z came first, and the first combining robots were
Getter Robo. (These were technically preceded by
Tetsujin 28-go, "Iron Man #28", better known in the West as
Gigantor, but it was radio-controlled rather than piloted and wasn't very influential on later shows)
Voltron (or
Beast King GoLion, to give it its original name) is almost unknown in Japan, since it was regarded as nothing special compared to the countless other shows about robots that were floating around. Ironically, when
Mazinger Z was dubbed (as
Tranzor Z) it was viewed as a ripoff of Voltron.
If you're thinking "those aren't as cool as Voltron", watch
this.
That reminds me, I have to watch some incarnation of Getter Robo some day. I mean, it's what Tengen Toppa Gurren Lagann was based on...