Superman, Batman and Robin meet the Composite Superman for the first time, from World's Finest (volume. 1) #142 (June, 1964). Written by Edmond Hamilton, art by Curt Swan, George Klein and Sheldon Moldoff.
Even as a kid I thought it was dopey that even though Composite Superman's mask was made of lead no one could figure out who he was - half his freakin' face is exposed! It would've been smarter to just explain that he used his Chameleon Boy powers to duplicate Superman's face (albeit in green), which he obviously did.
With all the powers of the Legion of Super-Heroes (including two Kryptonians and a Daxamite), Composite Superman was ridiculously overpowered. The only reason he didn't kill Superman and Batman and take over the world is that his powers soon faded, as well as his memory of them.
