In a perfect world, Unreal Engine of course, with good sound production, complex open world, multiple plot lines and characters... After programing in Ren'Py and DS modeling and rendering, I don't think we're going to see that any time soon. Unity is probably a close second and used pretty well in some cases.
Ren'Py just sorta works and the rollback function just works as long as the developer isn't dumb enough to disable it. Seems like the best Unity games are just mimicking Ren'Py, among the ones I've played of course.
Each of my game engines is a beautiful snowflake, and I try to make time for each of them in my life, so they don't feel neglected or think I'm playing favorites.
It's not my fault that Renpy and I have so many interests in common. She's such a daddy's girl.