Yeah, realistically it is PS1 even though you could make an argument for PS2 as well. The PS1 in Japan had hundreds of highly experimental, artsy, oddball games that never got localised and would never get made in any other era - plus the countless 2D games that Sony refused to allow to be released in the West. Famicom, Super Famicom and Saturn have a lot of unknown titles as well, but none of them are anywhere near as close to unique.
Here's the thing with answers like the DS and such, most "hidden gems" you're going to list are something hardcore gamers already know about. None of those games listed are particularly obscure (beyond LRRHZBBQ), all of them got English releases. When we come to PS1 you're talking tiers of obscurity far beyond that - stuff like Pop'n Tanks, Majokko Daisakusen, Love and Destroy, Running High, Milano no Arubaito, Kyuutenkai Fantastic Pinball, Kyoro-chan no Purikura Daisakusen etc. (and those are all entry level obscure for the PS1, shit gets even deeper than those).
I think it's the DS. It even has a meta thing going on in your poll, where the first two times I was looking at your options I completely skipped over it. It is the hidden gem of your hidden gems list. Here is my evidence:
Contact
Bangai-o Spirits
Henry Hatsworth
Feel The Magic
Little Red Riding Hood's Zombie BBQ
Retro Game Challenge
fucking Soul Bubbles
Lock's Quest
DS lovers feel free to add more to my list
PS2 I see you though with Mister Mosquito and Mad Maestro!
Here's the thing with answers like the DS and such, most "hidden gems" you're going to list are something hardcore gamers already know about. None of those games listed are particularly obscure (beyond LRRHZBBQ), all of them got English releases. When we come to PS1 you're talking tiers of obscurity far beyond that - stuff like Pop'n Tanks, Majokko Daisakusen, Love and Destroy, Running High, Milano no Arubaito, Kyuutenkai Fantastic Pinball, Kyoro-chan no Purikura Daisakusen etc. (and those are all entry level obscure for the PS1, shit gets even deeper than those).