Bags

Although there are many onebaggers who go below the 25L range, most of the time any decent normal backpack can fulfill their needs. There are also those onebaggers who use wheeled cabin luggage. Again, there are many designs which fit their needs. However, the story changes when we look at the 25L-40L range of backpacks, where requirements of comfort, clamshell (front) loading, durability, style, and weight come into play. There is a limited amount of backpacks that fulfill this criteria.

Therefore, this page will list down the most frequently cited backpacks, along with base specifications. A link to their own page could have some quoted reviews on pros and cons of these backpacks.

General requirements to put a backpack here: Note: If there is a particularly good bag that many onebaggers use, that does not fit this criteria, go ahead and add it in, and justify it in the linked page.
 * 1) Preferably front loading (clamshell/panel loading)
 * 2) Must fit European carryon sizes. I know the US domestic carryon sizes are much bigger
 * 3) Between 25L - 45L in size (The limit is the European carry-on regulations)

Carry-on sizes of various airlines around the world
https://www.carryonbagsizes.com/ by Minaal

Generally speaking, it seems 95% of airlines will allow a free carry-on bag that fits the IATA standard, which is: Most airlines allow bags a little bit larger, though some allow that in height, others in length, and others in depth. Only a very few super-budget airlines limit it to a free much smaller cabin bag. For those few budget airlines, if you want the full size carry-on, you'll have to pay.
 * 55 x 35 x 20 cm.
 * 21.5 x 13.5 x 7.5 in.

Common bags used by Carry-on only onebaggers
I've also come across a onebag google spreadsheet that someone (sologuides?) made.