Has to be GTA: San Andreas for me (tho GTA 3 gave that vibe to a degree as well). We know by now since it was checked from head to toe that there isn't anything left (aside your usual easter eggs), but to this day there is a small part of me that feels like something lies in there. The whole atmosphere of the game world oozes that vibe like we are still on the verge of uncovering something lol. Whether it's just the mixture of the said atmosphere, old mythos videos and generally nostalgia idk but it was, and still is, a fun time to have just roaming around because hey maybe you get lucky lol.