Standard utility knife that takes standard/inexpensive blades, with a handle shape suited to your preferred grasp of it and hand size. Pick folding or retractable.
You can get a knife sheath for your belt that fits just about anything, doesn't have to come with or be intended for a particular knife.
Otherwise many of the normal pocket knifes out there would suit this use, if you don't mind sharpening them every day (depends on whether you're just opening and breaking down boxes, or have to actually cut the cardboard itself which dulls a blade pretty fast), maybe something like a small Spyderco flipper (or a generic clone that costs 1/10th as much but dulls 3X as fast)?