While it is not a game itself per se, a game experience I'd love to relive would be the Joint Pak modset for Stalker.
The basics of the mod is it adds a massive amount of content to the game, such as weapons, maps, new factions, and enough quests to last several hundred hours. Unlike other popular mods like analmeme, everything is actually hand crafted and not procedurally generated "kill x of y" quests like all those shitty freeroam mods.
While the main quest starts out with just SoC, it actually gets interesting enough to stay engaging, with many branching and intertwined questlines. The mod expects you to do every quest instead of just pick and choose, which may be a bit much for some people. It has the single best sense of progression I've ever felt in a videogame, from a loser starting with an empty pistol to a demigod over the course of the 500 hour playthrough, with every incremental step in between and a challenge level appropiate for your strength, even at demigod levels.
I had only beaten it with 100% completion just a year ago, and the playthrough itself took several months, yet it always calls to me to come back and replay it.