Good idea to get our standards straight before starting the discussion, @mirv!
Now, my thoughts on screen resolution are as follows (it'll be a bit long, you can skip to the end for the conclusion or open the details pane below to see my reasoning):
Helios' wordy and extensive reasoning
I think the relevant resolutions in this day and age are:
Full HD (1920x1080)
Quad HD (2560x1440)
Ultra HD (3840x2160)
As I'm a fan of 16:10 screens, adding a bit of extra screen that's really nice for productivity work. I'd like to add:
Wide Ultra XGA (1920x1200)
Wide Quad XGA (2560x1600)
Hate on Apple all you like, but I like the 'retina display' concept: Once you can no longer make out individual pixels at a typical viewing distance ('typical' for a notebook is considered 20"), pixel density is high enough and a higher resolution at that screen size makes little sense.
With that in mind:
FHD and WUGXA are suitable enough for displays up to 13"
QHD and WQXGA are suitable enough for displays up to 17"
UHD is suitable enough for displays up to 20"
Alternatively, if you want to go for a 2-in-1 you may consider tablet mode (where 'typical viewing distance' for these purposes is considered 15"). In this case you'll need a slightly higher PPI.
With that in mind:
FHD and WUGXA are suitable enough for displays up to 9.5-10"
QHD and WQXGA are suitable enough for displays up to 13"
UHD is suitable enough for displays up to 19"
If we're looking at a device with a 13" display, QHD/WQXGA should be enough to give an excellent user experience in tablet mode, and FHD/WUXGA should give an excellent user experience in laptop mode, and still a very good experience in tablet mode.
If we're looking for a display size in the 14" to 15" range, one might make a case for an UHD screen for ultra-crisp close-up tablet mode viewing. At the same time, QHD/WQXGA would be very acceptable in close-up tablet mode, and more than sufficient in laptop mode.
Since battery life is influenced by resolution --with higher-resolution displays generally sucking more juice-- I think a case can be made for keeping the resolution as low as required. From this perspective I'd go with the laptop mode resolutions.
Display sizes and resolutions to consider then would boil down to:
If 16:9 has preference:
13" at 1920x1080 Full HD
14-15" at 2560x1440 Quad HD
If 16:10 has preference:
13" at 1920x1200 WUXGA
14-15" at 2560x1600 WQXGA
So pick your preferred display ratio. Then pick your preferred display diagonal. There you go, the most sensible resolution for your screen!