Dark Souls for me. I almost didn’t pay it any mind because I sucked at first, but eventually I found a build that suited my play style (dexterity build with the Great Scythe and archery), and I played through it about a dozen times since then. It was such a new concept for me at the time because I was so used to Skyrim and Fallout, and I approached it that way at first. It also prepared me to play Witcher 3 since the combat really reminded me of that (plus the difficulty at first was a decent hurdle to get used to).