You may be asking like, what´s so important or crucial about finding ,,the best" carry on? Isn´t it fine to just grab the first one you find, with the ,,carry on" label on it? Oh no, not exactly. There is way more behind it than you may think, and even something that small can make your holiday uneasy or even ruin it completely. How? By denying you from you flight, due to the wrong measurements of the luggage.
Unfortunately every airline may have different sizes. The biggest different is between Europe and America and also some airlines give more pressure on that subject (Easyjet did not allow me to have carry on luggage and a handbag - had to have literally three layers of clothing on me to have only one bag)
Sizes and prices
The most common size is 55 x 40 x 20 cm and weight is somewhere between 5 to 10 kg. What you have to be aware of, is buying your suitcase by a non authorized seller. By that i mean for example Samsonite - they specialize in this field and so they know what they talk about (and you know what to expect). But if you want to get your luggage in a supermarket (it is cheap and you are not a frequent traveler i totally understand) be aware of the sizes, as airlines count even the wheels! some stores give only the size of the luggage it self and don´t count the (sometimes) huge wheels. Those can get you off the flight and that is really not a good start of your trip.
Destination and health
How is this related? well if you are about to walk a lot afterwards, maybe expect the roads to be muddy or take some off-roads, then it could be more convenient for you to buy a backpack. On the other hand, when you have trouble with your back, or you expect it to be really heavy, you will be better of with a classical carry on with wheels. Some backpacks can also have wheels. Would that be the win-win for you?
How many wheels?
I swear on two wheels any time, as their endurance is way better. Also they are embedded in to the luggage and so they do not take any centimetres of the limits. The hidden two wheels can also fit better in to the overhead lockers. You may not look that cool strolling down the airport with just two wheels, but trust me that they will be way better when dragging it behind you, for example on the Prague´s cobble streets.
Soft or Hard shell?
Well both types have their pluses and minuses. I personally had both of them. My soft one had something leaked on it and smelled really bad afterwards and my hard shell cracked - they do not treat it really nicely, when loading to a cargo hold (if there is not enough space on board they can take it down yes.) On the other hand i have to say, that i am glad for my hard shell each time, when i see some people on board stuffing their things in to the lockers. So your belongings are more protected that way. Just make sure that the luggage is not too heavy, as it would take of the limit.
Other things to consider
It is always good to be organized and so some cube sets, can really make it easier for you to pack and know where you have what. They can also compress your clothing, so you are able to carry more than with a classical packing skills.
Get yourself a hand scale, you won´t regret. Some airlines do not have the weight policy on a carry on, but others do and it differs from 5 to 10 kg - quite the range i would say - where some may even charge a ridiculous amount for excessive weight!
Toiletry bottles are also something to keep in mind. It is better than to have your big bottles, plus there is another restriction of 100 ml, so keep your original products at home and pour a little into such travel bottles.