(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.
My heart says it should be Dark Souls, but my head reminds me I absolutely hated it until I'd figured out that the run from Undead Burg bonfire to Taurus Demon couldn't be rushed, and the point of the game was to be slow and methodical. Then I loved it.
For me it's the fact that it took me YEARS to git gud. Bought Demons souls before the hype, and had no clue. After a couple trys it went on the shelf. Went on to buy every souls game, cause I knew they were amazing, but never finished one or even got half way.
It was watching walk throughs, speed runs, and various challenge runs on youtube that, along with hours and hours of grinding, got me comfortable with these games.
Playing DS1 with fresh eyes and my current skills and knowledge of the other games would be amazing, as it really has the best world.
Having said all that, my answer is the Mass Effect trilogy.