Here is a compilation from this thread. Thanks to everyone for the input. If you have anything that fits in between or even topples the number 1 spot, please update the ranking accordingly.
Only condition is that bags are roughly comparable in size, i.e. 41-45 linear inches. Note that if you go from a foldable to a duffel or backpack format, weight tends to go down for the same outer measures, as demonstrated by the Eagle Creek and Osprey which I have inserted as a reference. For more info on each bag please see previous posts. For our American friends: 1oz = 28g and 1lb = 453g
Rolling:
Travelite Delite - 1900g (semi-hard) ca. $150 but not available in the US
Landor Hawa Sub0G - 2177g (soft) ca. $100
Heys X-Case-NS - 2182g (hard) ca. $90
Antler Wheeled backpack - 2200g (soft but quite structured) ca. $135
Non-rolling:
Up to 2lb:
A. Saks 21" exp. - 545g $52
Campmor Essentials - 793g $33
Rick Steves Classic - 907g $80
Up to 3lb:
Eagle Creek 20" Sport companion duffel - 1018g ca. $80
Patagonia Burrito MLC - 1151g ca. $120 (garment bag)
Tom Bihn Aeronaut - 1230g $200
Patagonia MLC - 1332g ca. $140
Rick Steves Convertible Carry-on - 1360 (3lb) ca. $90
Up to 4lb:
Osprey Porter 46 - 1416g ca. $100
Tough Traveler Tri-zip - 1500g $270
Tom Bihn Tri-Star 1550g (smaller than Aeronaut but heavier) ca. $240

Easy Going Carry-on - 1636g ca. $185 if you can find it
Ebags Weekender - 1640g ca. $56
RedOxx Airboss - 1770g $225
Or, the royal solution, A. Saks 21" expandable bag plus Samsonite Micromover cart (1007g/ $40) coming in at 1552g together. This makes it the only rolling solution under 4lb, under 3.5lb to be exact. To top it off it is also under $100 and the wheels can be used for other things or the bag carried without wheels.
It is interesting to see that there are really good alternatives to the hyped Bihn and RedOxx brands. Sure, the warranty might not be as good and the quality might not be as high.
But the A. Saks is three times lighter than the Airboss and costs more than four times less.
Rick Steves Classic weighs only 2/3 of the Tom Bihn Tri-Star and costs only 1/3. Those are hard facts to consider.
And at a fabulously low $33 and less than 2lb. the Campmor Essentials is a totally painless entry into ultralight luggage and onebagging! At $56 the ebags Weekender convertible is a fantastic deal for such a full-featured and lightweight bag, too.
My personal favorites would be the Travelite for the wheeled, the A. Saks for ultralight, the Patagonia Burrito for garment bag and the ebags Weekender as a convertible.
Till