In other words: if there is a person that owns that device and is willing to develop/port/maintain Ubuntu Touch for the device. Knowing that, which of the current devices has most developers dedicated to it?

Volla company have team of people working on support. they even sold their devices with Ubuntu Touch pre-installed. (And it is a German company and Germans wants quality, so they cannot afford to sell crap). so all Volla phones is and will be supported probably the best.

some core developers also have Fairphone 5 (some even a Fairphone 4 somewhere) so that will be supported well too.