I just watched the newer anime. I don't have a burning desire to go back to 70's manga and read through decades of changing art... or the inevitable hilarious translations of old anime. xD
Armageddon? Shin vs Neo? Or New Getter Robo?
The manga is actually remarkably consistent in style and has amazing art.
Getter Robo and Getter Robo G have a Nagai style going for them, except for when Ishikawa suddenly draws something in his own style without warning only to change back later. Go sticks to that one art style and runs with it.