For me the best is a 2-wheeled roller bag that weighs 5 pounds or less empty. All my bags fit that basic description. I can pack one like that with all I need for a trip of any length in any season. Many folks swear by backpacks, but even though I pack light, I found the one I had bulky and cumbersome to put on, take off and wear. If you explore the Packing forum, you'll find lots of pros and cons for backpacks and roller bags. I've found roller bags much easier to manage than a backpack and if you get a good, lightweight one you can always pick it up by the handle on the side (not the expansion one for rolling on the top) and carry it that way if needed.
My trips tend to be 4+ weeks. I've been traveling solo since 2016, so I must be able to manage my "big" bag on my own. I do carry-on almost exclusively and limit to 20 pounds fully packed. I prefer the Eagle Creek and Osprey brands, but there are several others that are equally lightweight and work well.
Both of my Eagle Creeks are expandable, but I've used that feature only once on a flight home from the UK in 2016 because expanding made the bag too big to carry on. The EC bag I used on that trip is US "standard" size, 22x14x9 with a U-shaped handle and a capacity of about 45L when not expanded. It's so old that I'm not sure of the capacity expanded. My other EC bag is called "International" and is 21x14x8 -- about 36L without expansion, or 21x14x9.5 -- about 39L with expansion and has a T-shaped handle. I've never expanded that one. I will say that an advantage of an expandable bag is that you can pack it expanded, then zip up the expansion for a tighter fit.
Neither of my Ospreys is expandable. Both have T-shaped handles. One is 22x14x9 with a capacity of 42L. The other is called "Global." It's 20x16x8 with a capacity of 38L. Notice the difference in shape. On my last trip I chose what I wanted to pack for 5 weeks in both Irelands and Wales. Then I tried all 4 bags in turn to see which worked the best for my compression cubes and other things I wanted to go in my main bag. The shape of the Osprey global bag was the best for packing and that's what I took on my trip.
Fortunately, I was flying British Airways and the size was well within their guidelines. If you plan to carry on, much depends on the airline's rules. Note that all the measurements I've given are in inches and include wheels and handles. Typically those are rounded up from the cm ones. That means that the bags are actually a bit smaller than the inches listed.
With only a 2-week trip, your temptation will be to try to pack so much that you don't have to wash clothes along the way. Resist it. The lodgings you're planning to use should all have laundry facilities in them or close by. Some might even do laundry for you. And of course you can do a bit of sink washing if needed. Contrary to some folks who think doing laundry in a self-service laundromat is a waste of precious tourist time, some of my most enjoyable cultural experiences have been doing laundry. ;-)
