Tough one, I would say Final Fantasy IX and Pokemon Gold/Silver/Crystal.
Final Fantasy IX, mostly due to nostalgia and it's a pretty great game if I may say so. It also got me into RPG genre altogether.
While Blue and Red were fun and addictive Gold/Silver/Crystal were a massive step forward, one this franchise hasn't done ever since.
Other games I hold in high regard, Dragon Age Origins & Inquisition, Final Fantasy XII, Gran Turismo 2, Kingdom Hearts Chain of Memories (GBA), Mass Effect, Metal Gear Solid, Pychonauts, Starcraft & Brood War, Tekken 3, TLOZ Link's Awakening, Quake II,... and quite a few others.
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