I’d like to say Starflight, as that was definitely a formative game for me. And I dropped so many hours into the various Infocom games that it’s hard to deny them, but for most engrossing I probably need to go back to Magic Candle, for reasons I’ve discussed elsewhere - and since I am repeating with this one, I’ll just link to my explanation.
Either that or Ultima V. I freely admit that IV is the better game, but it didn’t grab me the same way as V. I needed to get revenge on Blackthorne and free the country in a way that I didn’t need to become the Avatar. Anyway, with Ultima V and Magic Candle both I filled notebooks with scrawlings about rune translations, reagent lists and locations, magic spells and what they could do, and many many maps. I am not sure I’m even capable of being engrossed these days the way I was then.