It depends on the genre and the implementation.
The obvious games that come straight to mind when exploration is mentioned are Elder Scrolls, Fallout or GTA. These sandbox games give you full, living worlds that you can truly immerse yourself in. In every area you go to there will be something that makes it worth exploring be it a quest, mission or simply an Easter egg placed there by the developers.
However, there are plenty of games that aren't pure sandboxes that have good exploration. The older Final Fantasy games, Dragon Age Origins, Bioshock and Mass Effect are examples of games that take you to a range of different areas in a relatively linear fashion, but each area has plenty of quests, gameplay and lore for the player to discover if they choose to.
Similar to this are games like Deus Ex and Dishonored where exploration is essential and the game actually gives you multiple paths to achieve the same goal. The gameplay and exploration go hand in hand with these games as exploration will often reward the player with new methods to complete a given objective.
Anyway, those are my faves.








