I'm not talking about true open world sandboxes, but more condensed game levels, linear or nonlinear, with just enough open-ness to tickle your exploration itch while doing the main missions?
My top 3 would be Prague in Deus Ex - Mankind Divided, Santa Monica in Vampire the Masquerade: Bloodlines, and the Freeport City Dam level in 1998's "Sin." The latter gives me some nostalgic Goldeneye vibes, just without the frustrations of playing Goldeneye.
What are your picks, new or old? Doesn't matter when the game was released.