A 10-foot USB cable. Most people use whatever's packed in with their electronics, and it's pretty rare to get anything longer than 6 feet. Having the extra length is really nice in many situations.
It's also one of my go-to inexpensive gifts or part of a care package for someone in the hospital. The extra length there is often the difference between being able to use a device while it's charging or not.
Eh, I have audio interfaces and MIDI controllers on 10ft cables cause shorter just don’t reach my PC, works perfectly fine. Longer than that is a gamble but as far as I know 10ft is the upper bound of the USB 3.0 spec, so should be totally fine unless you have especially shitty cables.
I have long ones too that run my old Oculus Rift sensors. Those can ocassionally be flaky but I think it's the controller rather than the device. I'm not saying they don't work, I'm saying they are less reliable and/or more expensive to make reliable. Hence companies not bundling them.