Looks like 185/65-15 is the stock size. Looking on tirerack.com shows you have 11 choices for LRR tires.
Tire Search Results
Out of that list the best tire as far as I'm aware (edit: see next post!) is the Goodyear Assurance Fuel Max tire ($85 ea). They used to cost a lot more, so I personaly went with the Bridgestone Ecopia EP100s ($78 ea) on my Prius about a year ago. Thus far I've been very happy with them even in snow. I know others who went with the Fuel Max and are very happy with those as well. The Goodyear Integrity tire was the 2nd gen Prius OEM tire. It has good rolling resistance, but not so great of traction (see review below).
When Round and Black Becomes Lean and Green
You might just want to give tirerack a call. I haven't talked to them in a while, but the tech support there is actually pretty good (on performance related issues). Since LRR is now a big deal their support probably knows a bit more than they used to.