What is the best carry on luggage?

Specific Situations

Trip Length - If you are going on a longer trip, most often you will need to bring more and vice versa if it is shorter. You should pick a suitcase that will fit what you need and nothing extra.
Souvenirs - If you plan on bringing items back from your destination, an expanding suitcase will provide an easy carry on outgoing, and a great check-in on the inbound.
Physical Ability - You want a suitcase that will not require you to bend over to far if you have a bad back for example. You will most likely have to lift it onto the TSA conveyer belt which would require properly placed handles. If you have a physical limitation, your luggage should make life easier and not harder.
Safety - If you are traveling to a poor country or a place that has a higher than normal crime rate you want to keep a low profile. Bringing a branded Louis Vuitton luggage will attract unwanted attention. Instead aim for a good suitcase with neutral colors like grey or black.
Personalization - Many people use their suitcase as a reflection of their personality and travels. Some decorate it with stickers, others are proud of the battle dents on their aluminum shells, and others choose colors or customizations like monograming. Whatever your choice is, make sure it does not detract from the basics above.
Batteries - Many suitcases come with a battery pack built in. You want to make sure it can be removed, as to not prevent you from boarding an aircraft with a specific battery policy. One thing to keep in mind is the space it takes up. Often times a separate battery pack will die you more options on placement, charging capacity and takes less room from the storage volume.
I hope this guide helps you choose a great carryon luggage. If your bags are always falling off, airport security is a pain and staying organized is difficult check out the Travel Bag Buddy