Webster's defines the word "best" as "better than all others in quality or value". This definition is both helpful and the cause of potentially even more confusion. It's helpful in that it links the idea of "value" with the definition of best. Most people can define value.
As for the confusing part, define quality.
The lead character in Robert Pirsig seminal road novel, Zen and the Art of Motorcycle Maintenance" literally loses his mind trying to come up with the definition of quality.
There is an old adage that is sort of an add hock definition of quality, "I know it when I see it". This came about because people lack a true definition. So without a true definition of quality, is it possible to answer the question about who makes the best mens leather bags?
(You were wondering where I was going with all that weren't you?) :-)
I'll try to answer the question this way. Let's take 3 leather bags, one from a vendor on ebay selling on discount, one from a small vendor selling on value and one from a world famous European designer brand selling on fame and their multi-million dollar ad campaigns.
Ebay has been flooded recently with cheap leather bags coming out of the over producing factories in China. Many of the bags claims to be "genuine leather" when they are using bonded or PU leather which isn't leather at all, just a form of vinyl.
These bags are usually $30-100 and if you have any issue with the bag, well you're on your own. So this bags scores points on value (maybe) but comes with questionable long term durability and zero customer service.
For our world famous European designer brand, lets use the Salvatore Ferragamo Mens Briefcase. $1800 of pure Italian opulence. For that much money it better last the rest of your life and look good doing it. High scores for materials, durability and hopefully, design but the value score plummets with a price equivalent to a decent used car. Customer service should be excellent as well.
For the middle of our test scale, let's use . They're a small company who has been making leather bags since 2007. Naturally dyed hides, hand made with a same passion and attention to detail as the Italians. Have a problem? Call them and talk to the president of the company. Try calling Salvatore and see if he'll take your call. Prices start around $200 depending on what you are looking for.
Like many have said on many of these threads, any one looking for the "best" of anything is looking for the best for them. Hopefully this answer offers some perspective on choosing the best for you.