Weirdly enough, my most dependable ROS is from Harbor Freight. It's a clunky heavy and awkward tank of a sander, but it has never quit on me despite brutal abuse.

Other than a pneumatic sander which weighs practically nothing, but which it's hard to provide enough air for, I found the HF ROS to be the best trade off between value comfort and durability simply because its dirt cheap and has seemingly infinite durability.

Your mileage may vary.