When they are brown and mushy.

Because thats when you can turn them into banana bread.

If I am going to just eat the banana by itself, I prefer it to look fake as fuck and bright yellow. Just ripe enough that the peel isn't super hard to remove.

I haven't tried frying one, tho. Might be better if it's still green for two reasons: Higher sugar content for caramlizing and firmer so it doesn't just melt away.