What is the best affordable power tool to get the lug nuts off my car tire?

Harbor freight sells a lot of tools that are pretty cheap, but keep in mind that you get what you pay for. Having said that, if you are just going to use it for lug nuts, it should do the job but I would recommend a socket for your lug nuts that is impact rated. Impact wrenched are what the tire shops use all day long to remove lug nuts, but don't use it to tighten them, you have no control over the torque and can break the studs easily. Always put them on the old fashioned way, or spend the extra money and buy a "torque stick" rated at the correct torque for your lugs.