Probably Shadow of the Colossus. On my first playthrough, it was a completely alien game to me, with only a little narrative other than ride horse to where the light points to, kill big mean-looking thing, repeat. Despite that concept, I was amazed by the beauty of the world as I rode on my trusty steed Agro. I would purposely get lost and ignore my current goal just to explore the massive environment, see what's hidden behind the next mountain, or through that next cave (sometimes stumbling upon the sight of a future boss location in the process without knowing it). And the Colossi themselves were one of a kind, as well. I think everybody who has played the game remembers their reaction to seeing that first behemoth stomping around before you, violently shaking the tree with every step, completely oblivious to your insignificant presence until you grabbed his attention. Figuring out strategies for a Colossus was also a part of the fun, sometimes having to think on the fly if it were always aggressive towards you. Subsequent playthroughs were still enjoyable, be it playing to beat a previous time, to unlock new items through Hard mode, or just to experience it again, but it will never capture that feeling of playing it all without any experience. Not to mention that by now I know the secrets, the location, the theories behind what little story we have to go by, just about everything. Shadow of the Colossus was a true once in a lifetime experience when it comes to video games for me, and one I would gladly play through for the first time again.
tl;dr if you're a jerk: I would like to yell "HOW AM I SUPPOSED TO KILL THAT!?!?" after witnessing the first Colossus again.