I remember playing the game and generally being

once i left Los Santos and realized i was still driving in the highway for 5-10 minutes...
I couldnt believe the PS2 could handle all that space
I'm a nerd so I knew at that time that open world games take up, generally speaking, the least amount of space in those times. repeating assets over and over.
I'm going to offer three answers here.
Most astonishing from a gameplay perspective.
GOAT Moment
For me, the gamer, just on some nostalgia shyt, it is probably Legend of Zelda, The Wind Waker, when you can finally walk around the original Hyrule Castle, following the timeline
For perhaps the defining moment as far as elevating gaming as an artform?
Would you kindly from Bioshock 1, contains spoilers.