I will say I got to handle a galaxy fold 4 and if a screen has to be bigger, I don't mind as much as long as it's narrow like that phone. I was still able to one-hand it comfortably.
Yup, that was the iPhone 6 screen. If Jony Ive’s dream of a glass slab would come true, a phone of that size (under 7mm thickness by the way) with a bezeless display would have been at around 5-5.5 inch. That’s what phones should be like now instead of horrible camera bumps and notches.
The idea of "best" for something that people have different preferences about makes little sense.
I'd like under 5.5" on a modern design with minimal bezels resulting in a device no larger than the Nexus 5. The fact that nobody's selling such a phone makes me think most people prefer something else. Everything is huge now, but some are huger than others, demonstrating there's still a range of preferences.
Again people, diagonals without aspect ratio is pointless, because there's a lot of variation on the sizes.
I think mine is 6" 16:9. The Galaxy Note 3 at 5.7" 16:9 was pretty damn close to my ideal. I still find phones a bit too small (read: narrow) for my liking.