Muteki Choujin Zanbot 3

Another sick, sad, high-bodycount series by the same director. 

Getter Robo introduced the combination (Gattai) concept of super robots, with three different pilots and jets able to combine into a bigger and stronger robot and Zanbot 3 is definitely inspired by this. Both plotlines follow the villain of the week structure with an alien race menacing mankind and with a share of dramatic moments on them.  

Zanbot 3 is also directed by Yoshiyuki Tomino, whose most notable work is the Mobile Suit Gundam anime. Zanbot 3 aired two years prior the original Gundam and many similarities can be found between both series, most notably on how the main character reacts to the menace they are facing and the way war affects the lives of those involved in it. Zanbot 3 paved the way for the Real Robot anime sub genre with its dramatic take on the typical fantastic and superheroic robot shows that came before it, thus, making it a natural recommendation for anyone looking for the origins of Gundam, Code Geass or Evangelion. read more 

Zanbot's companion series; A bit lighter in tone but it still has a grim ending. Again, directed by Yoshiyuki Tomino. 

Director Hiroyuki Imaishi admitted in an interview that Zanbot 3 was one of his influences on making Gurren Lagann. Thus, if it's good enough for him, it's good enough for you. Throw your prejudices of old animation to the curb and embrace the awesomeness.