Far Cry

While maybe you could argue Halo was the first open world shooter, I'd argue Far Cry is really the first one. It played like a true simulation. You had lots of guns and vehicles, some objectives, and you could kind of go off and do what you wanted. And it was incredibly fun. I think it basically had no story, no real reason to be doing what you were doing but the sandbox nature of it was incredibly fun. I remember just being blown away that I could get in a boat and just... ride off to different island and fight or avoid who I wanted.

It's crazy to see how far it's come since then as a series but this is the one that started it all.