Well for PS1 my Hidden Gem recommendation is "Machine Hunter"
This is just gameplay of the first level, no commentary. No one I follow on youtube has really reviewed this, or if they have they might mention it briefly in a 20 minute video.
It's a very neat little co-op top down shooter from the early 90's. D-pad controls which way you move, while the buttons signify which way you shoot. So if you press triangle, you'll shoot up, and can use the d-pad to move around while shooting up. If you press X you'll shoot down. Naturally square and circle will fire left and right.
The main draw of the game is, as you might see in the gameplay, you can come across these robots that shoot at you. However damage them enough and they'll basically become this faded orange color and have electricity spark out of them. This allows you to walk onto them and possess them. Taking on their form and abilities. It doesn't get too crazy, it's usually just mobility and weapons.
Every time you possess a new robot, you get a full life meter. However you need to be careful. If you keep shooting them while they are in their 'possess me' stage, you CAN destroy them completely.
You can also see in the gameplay them getting level ups to their firepower. There is the green level 2, and blue level 3. Sadly using them depletes a meter and you'll go back to level 1 after a bit, but thankfully you can toggle the extra firepower off and on.
Levels pretty much have objectives you need to complete. Like the second level has you looking for bombs to disarm.
It's a really cool game to mess around with, but naturally being an early 90's game it's not great by today's standards. The missions can sometimes feel dragged out, and the controls aren't the greatest, but this was WAAAAAAAAY before twin stick shooters. Heck this was before we even got twin sticks!
It has some neat environments to explore though. Later you end up in a hospital area where the top half of corpses will crawl at you, leaving a blood trail behind them. Pretty sure when you walk/ drive through blood it sticks to your feet/threads for a bit too. Just a cool little relic from the past, but one I do suggest at least checking out if you can.