The weirdest I've ever seen, anyway. 70's Robot Anime Geppy-X is a PS1 game that mixes sidescrolling shooter gameplay, Super Robots, and frickin' toy ads. The whole thing feels just like what the name suggests: a Super Robot show from the '70s.
As you can probably expect in a game with that description, this is Japan-only. Still, I gotta know: has anybody here played this? I'm not a hardcore SHMUP fan but this seems way too weird to pass up.