A good travel backpack make sure you will never want the heavy wheeled luggage. They have an internal-frame backpack and come in the 40- to 60-liter range, so you can keep all your stuff in them, and are sturdy enough to be carried wherever you go.
Good travel backpacks are big and can support a lot of load across long durations. They are designed to provide easy access to your stuff, as well as decent weather protection, and are light.
When choosing backpack, start with fit. Always buy a backpack that fits your body.
The best travel backpack for men and tall women: Osprey Farpoint 55 M/L
The best adjustable, comfortable travel backpack for normal women: REI Grand Tour 80
The second best travel backpack for men: REI Grand Tour 85 (very, very roomy)
The second best travel backpack for women: Osprey Farpoint 55 S/M (for all-around use)
The best affordable travel backpack: Highlander Outdoor Explorer Ruckcase 45+15 (great price, some compromise on all-around utility)