In terms of just calories per ounce, French fries, but in terms of what people usually eat, the answer would be chips. I have links for McDonald's, to get their fries and for Ruffles and Lays potato chips and lastly Baked lays.
Fries are about 95 calories an ounce
Ruffles are 160 an ounce
Lays are 150 an ounce.
Baked lays are 120 an ounce
All of the chips serving size is an ounce approximately 10 chips. Lets say that most of us eat 2 ounces max, so about 20 chips.
McDonald's small fries serving is 2.6 ounces and not many of us get small fries.
250 calories for small, 380 for medium, and 570 for large, not too mention this does not include calories for ketchup, 15 calories per ketchup packet.
So my bottom line is most would be better with chips, unless you can get by with small fries and no ketchup.