The first thing to consider when choosing a backpack is the size. You may be tempted to buy a large backpack: maybe 50-80 liters. This is almost certainly not what you want. You want to think about packing as light as possible, definitely under 15kg in your pack. The best backpack for traveling is probably 30-40 liters, which allows for everything you need and stops you from packing extraterrestrial things which you'll use and will hurt your back.
The best backpack for travel will also be light. Surprisingly, many backpack harnesses and materials are unnecessarily heavy. You need to consider the durability of the material against the type of use you're likely to put it through. Some ultra-light materials can be very durable but will rip if worn in an airport conveyor belt. If you're flying often consider tougher material.
The best backpack for travel must be able to be locked. If you have a hiking-style top-loading bag, it's unlikely to be easily locked. You may need another "overbag" to cover and lock it for flights, left luggage, and when you leave it behind in your hostel or hotel room.
The size and shape of your backpack is probably the most important factor, however. There's no amount of reading that can help you with this: you need to try the bag on, with some weight in it, and enlist the help of a friend or shop staff to fit it correctly. You might find the harness does not sit right on your hips or around your breasts. You may find the length is too long or too short – creating pressure in the wrong places.
At the end of the day, the best backpack for traveling around the world is a personal decision, but you can learn a lot by reading travel blogs and asking the people who have been traveling for a long time. There's a lot of people willing to help with your travel plans.