The first thing to consider when choosing a backpack is its 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, certainly less than 15 kg in your backpack. The best backpack for travel is probably 30-40 liters, which contains everything you need and keeps you from packing strange things that you will rarely use and that 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 are likely to endure. Some ultra-lightweight materials can be very durable, but will tear if caught in an airport conveyor belt. When flying, often consider harder material.
The best backpack for travel should be lockable. If you have a hiking style top loading bag it is unlikely to lock easily. You may need another “overbag” to cover and lock it for flights, left luggage and when you leave it in your hostel or hotel room.
The size and shape of your backpack is perhaps the most important factor. There’s no amount of reading that can help you with this: you have to try on the bag, with some weight in it, and enlist the help of a friend or store staff to fit it correctly. The harness may not sit properly on your hips or around your breasts. You may find the length too long or too short, creating pressure in the wrong places.
Ultimately, the best backpack to travel the world is a personal decision, but you can learn a lot by reading travel blogs and asking people who have been traveling for a long time. There are many people willing to help you with your travel plans.