Man that would be a tough decision.
I would go with Star Ocean 2 on PSX:
1 of my all time favourites, the music is incredible for a start, just classic Enix. The combat was incredibly good , fast-paced, aggressive and real-time, but also required strategic planning.
It is the perfect universe (quite literally) to get lost in on a deserted island with plenty of time on your hands and time you will need to beat the game 100%, as you will need to play it several times to enjoy the full cast of characters and unlock all the personal events, go through the end game optional dungeon, reach level 255 etc...
The crafting system, the skills, the arts, everything is so deep and interesting.
Just thinking about it now just reminds me of of that first encounter between Claude and Rena in that village on Expel 25 years ago, the eerie music, little did I know of the incredible adventure I was embarking on.