It ain't that retro, but definitely Dark Souls 2 Scholar of the First Sin and its sequel Dark Souls 3. These were the 2 main games I would always boot up on my PS4 because they were just that addicting. I loved the gameplay loop, the different areas to explore, the atmosphere and feeling of dread every corner, and of course the combat. They're both drastically different as 3 feels more like a real successor of the 1st game, while 2 SOTFS feels like a reimagining of Demon's Souls. I ended up filling up all 10 character slots for both games with different builds, achieved the platinum trophy on both, and beat each of those games about 12 times each. I even beat 2 without resting at a bonfire to nab a special ring meant to show off your bragging rights. Dark Souls 2 SOTFS I also reached above 1000 hours with 3 having about 6-8 hundred.
As for retro games I could play forever, it would have to be classic Mega Man. I have beaten Mega Man 2, 3, 4, and 6 numerous times as a child as those were the ones I revisited often (I didn't like 1 or 5 as much, never beat 7, and 8 I only beat once). Something about the platforming, charming graphics, and gameplay loop has been my favorite of the series. About 20 years strong of playing them too.
Then of course there's Pokémon as at least one game in each generation up to gen 7 I've beaten at least 3 times. Gen 3 specifically I've beaten each game in it at least twice. Always using different teams and strats to shake up replay value.