Look for tires with a quiet/smooth ride, unaggressive tread, and high maximum pressure rating.
Also, ask for tires for the Prius, which uses specially-made LRR tires.
Often that describes the cheapest tires on the wall as well, and longer treadlife claimed does not always mean stiffer compound / lower rolling resistance. It usually indicates a lack of plasticizers used to extend tread life.
"If you want to save gas I suggest you permanently remove the drivers seat and steering wheel. That seems to help." -Oscar Halverson
Does a stiffer compound mean LRR? If so, then I'll once again tout Bridgestone "Turanza T", which I abused for 70,000 miles and still had lots of tread, and which handled/rode well at their max 44psi when the car I used them on recommended much less.