Tomba! (or Tombi!), one of my favorite PS1 games growing up. It’s essentially a metroidvania done before the -vania part truly cemented the style: lots of items to find, lives to lose, mandatory and optional abilities to get, back and forth opening every chest once you get the keys.

The game got a Steam release some time ago

I also have a sweet spot for Mega Man Legends 1 and 2, though I don’t think they’d count as obscure.