Megaman X. I was never a fan of classic Megaman, but the faster, more action-oriented sequel/spinoff X series rates amongst my favorites. It has tight controls, good music, varied stages, and memorable bosses and combat encounters. I must have beaten the first game dozens of times over the years.
The Legend of Zelda: A Link to the Past. It and Link’s Awakening on the Game Boy were so close to perfect that decades later they’re still the basis of comparison for any new 2D Zelda-like.
PC:
Baldur’s Gate II: Shadows of Amn. it was the game that introduced Bioware’s trademark party banter and focus on interesting and likeable characters. The systems are a little rough but it still mostly holds up. Though it’s been a while since my last playthrough, and I usually stop once I hit the Underdark and the open world structure constricts for a few hours.
Super Nintendo:
PC:
The music in the SNES Megaman X series is magic.