There's a lot of options so from least to most:
Kumatanchi: solely because it's the only Vanillaware not to be translated
Star Trader (X68000): It's a mix of shmup and RPG, which makes it a pioneer decades before Undertale, this could be historical. But other than that eh.
the other Starfy games: I'm mixed on whether or not I want to try them, but the fact that Nintendo dropped these games (unlocalised) on Switch could mean something? I guess?
Giten Megami Tensei: gorey and likely ancient as HELL but important. All the way down here because the translation has made good progress and could actually happen soon
Etrian Mystery Dungeon 2: ain't right to have an EO go untranslated
Persona 1 JP PSX: The true definitive way to play Persona 1. Has a few unique features that PSP didn't retain, shame.
Yakuza 3 JP PS3: The true definitive way to play Yakuza 3 as it doesn't have a few of the issues Remastered has (like the weird quickstep spamming a lot of enemies are doing) and all the missions.
Yakuza Kenzan: Only game to not be translated at all and SEGA has no interest in localization
the remaining Sakura Wars games: well this one's obvious
Grounseed (PC-98): solely to see where the fuck Shop Demo plays but also because the music is really good in general
Mime (PC-98): See above, it's kinda crazy Wish appears to just be a cutscene theme
Black Bird (PC-98): see above + it's a dungeon crawler
RONDE (Saturn): Because it would be funny and Marsh would finally make "The RONDE experience"
Devil Summoner (Saturn): obvious reasons again
Code R (Saturn): I hear it's shit, but a mix between racing game and dating sim makes me curious
Tales of Destiny 2: no reason needed. Also applies to Hearts on DS and Innocent R
Custom Robo V2: I NEED IT
Ore Shika (PSX): I think I've sung this game's praises and explained my point well enough multiple times
Segagaga (Dreamcast): Name a reason NOT to play Segagaga. SEGA didn't even bother making an anime girl for it in the mobile game where SEGA was EVIL. FOR SHAME. Absolute shame.
(image limit is 15, I swear it used to be 20 at one point)