Cotopaxi Allpa 35L Travel Pack, available at Cotopaxi, $220
Plain and simple, this is the best carry-on bag I've ever found. Ever since Cotopaxi sent me a test unit almost five months ago, I haven't gone on a single trip without it.
It fits a lot, while still meeting airline restrictions for a carry-on, and since Cotopaxi is technically an outdoors gear brand, the bag itself is reminiscent of that. The Allpa inherited innovation from that space, like a low-profile harness that helps evenly distribute the weight of the pack so it's comfortable to wear even when stuffed to the brim.
It also opens like a suitcase, so you don't have to waste time digging to the bottom of a traditional backpack.
There are many great selling points to the Allpa, but for a frequent traveler, it's the versatility, durability, comfort, and roomy design that are perhaps the best of the best.