Well, it should be
light weight luggage
, yet sturdy and affordable. It could be soft-sided (which expands easily to accomodate purchases you make along the way); or hard sided.
No matter how you travel, you'll have to lift and cart your bags at some point. The stuff you pack can weigh a lot; you don't need to add to that with a heavy bag. And aside from the potential damage to your back from heavy luggage, airlines charge $50 or more for bags that weigh over 50 pounds!
You can protect yourself from excessive baggage fees with light weight luggage, and a good luggage scale.
Luggage scales
are easy to use, and small enough to tuck into your suitcase or carry on so you don't have to pay fees coming home, either (after you've stuffed all those souvenirs into your bag!).
The best travel luggage also needs to roll smoothly without tipping over when you hit a bump or need to hurry through the airport. Spinner wheels, or those used in roller blade skates, work wonderfully. Many bags have 4 wheels for added stability; you can often pull those bags along at your side, without extending the handle. But even a bag with 2 wheels - if it's well designed and has good-quality wheels - will be an asset when you travel.
The ability to expand the bag is another asset for those who love to shop on vacation. I try not to overstuff my suitcase when I pack for a trip, but I always end up buying more than I intended! A zippered extension - even just an inch or two - can work wonders in getting it all in without having to buy another carry on tote. Of course, this is where you'll really need that luggage scale! You might have to re-arrange items in your checked bags to make sure none of them are over the limit.
You'll find a wide variety of good-quality suitcases, carry on bags, and sets on the pages below: