Open world sandbox is probably my very close second favourite genre, and I think it's fair to say that Rockstar set the grounds and bar for it. Needless to say, the GTA series are a huge part of my teenage/adult gaming.
[GTA series], [Sain't Row: The Third], [Minecraft], [Terraria]
And finally, story-driven RPG's are close to, if not tied, with sandboxing when it comes to how much I love it.
[The Legend of Zelda; Series], [Dragon Age], [The Elder Scrolls: Morr/Obli/Skyri], [Mass Effect], [Deus Ex: HR], [Fable: Series], [World of Warcraft] ... I'm definitely forgetting some. I Ill add more as they come to me.