It's a very difficult question to answer. Best of all time what? RPG's? Sandboxes? MMO's? They're not comparable at all. For example, I have SO MANY fucking hours in Rimworld, but I would never say it's a 'better' game than Disco Elysium, where my hours played are very limited indeed (it's not a game you play for weeks on end, of even replay very often). They're both good games, but I would still say Disco Elysium is a 'better' game, even though I would never pick it in a "you're only allowed to play 5 games for the rest of your life, which do you choose?'-situation.

So let's see, I guess any 5 of these would do (Mass Effect + 4 of these anyway)?

Mass Effect
Disco Elysium
Kingdom Come: Deliverance
Path of Exile *
Deus Ex
Rimworld *
Terraria *
The Witcher
Baldur's Gate
Dishonored
Divinity Original Sin
No Man's Sky *
Slay the Spire *
Fallout New Vegas

And an honorable mention because I played it for over a decade in the past:

Ragnarok Online

Honestly, I've tried so many MMO's in the years since then trying to capture that feeling.. You all know what I'm talking about. THAT FEELING. Never found it again.

(The games with the * are the ones that I have the most hours in, not because they're the "best" games, but because they have the most replayability and no stories that can end. In a "pick 5 games for the rest of your life" situation those would be the ones I'd choose, I think. Or maybe I would trade one in for Mass Effect, because I can't have N7 Days otherwise. ;))