(Saw this question asked on another popular link aggregation website and it got me thinking)
If you could play one game for the first time all over again, what would you choose? This might be because you want to do it all again, or because you don't think you got enough out of it the first time. It could be experiencing the game exactly as you were back then, or experiencing a game with what you know now.
For me, it's Legend of Zelda: A Link To The Past, experienced exactly as I was back in 1991.
Nothing comes close to how jaw-droppingly amazed I was by that opening sequence. The epic orchestral score, the cinematic rainstorm, creeping around in the dark... it was a generational leap above anything I'd played on 8-bit computers and consoles, and even the Megadrive. I'd love to play it again without thirty plus years of Nintendo/Zelda knowledge, or without knowing about the dark world.
For pure amount of enjoyable play hours: Skyrim, hands down.
For emotional/philosophical impact: Either Journey or SOMA. Both had a big impact mentally for me.