Least enjoyment would just have to be whatever is the most expensive game in my endlessly growing backlog. The other half of this question is more interesting, so I want to give multiple answers.
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I've got just under 4,200 hours in Terraria and it's still my favorite game! For the $10 it cost me, that's 420 hours per dollar. Nice.
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Minecraft would be a close contender. There's no way to track my hours in it, and I don't play it so much anymore, but I have to have at least somewhere close to the 4k I've got in Terraria, if not more. This game shaped my entire childhood back when the Xbox 360 edition first came out.
Those two are great, but they're also some of the best selling games of all time so I'll note some more niche ones as well.
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DLC and all, Melvor Idle is a game I spent $25 on (edit: though the full base game is completely free if you don't mind playing through your browser) and managed to put just under 3,000 hours in. Now admittedly this is an idle game, so a large chunk of that time was having it open in the background while I do something else. But it's surprisingly interactive and I'd like to think at least 1,000 of those hours were me either actually playing or planning.
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Barony is a game I've put 650 hours in since 2018, and man has it come a long way. Including DLC I spent about $30 on it for myself, though I've also bought the game or DLCs for multiple friends so its probably closer to like $60. It's a permadeath roguelike with next to zero meta progression. Check it out if you're into the genre, its currently on an all-time-low sale for $2 which is a damn steal.