I've switched to Mobil1 5W-30 in my mom's Prius. I've used Mobil1 in my Honda Rebel 250 and it has gone a phenomenal over 90,000 miles without any problems, but that was 20W-50, which is definitely not what you want to put in the Prius.
However, for my Prius I've done a bit more research and I'm using Pennzoil Platinum 5W-30 in it.
Of course, both are synthetics. And unless you live close to the arctic, I think you should stay with 5W-30, not switch to 0W-20 or 0W-30, because you could be opening up a lot of questions if you ever need warranty work.
From what I've read, the Mobil1 at 30 weight is within the specifications to define 30 weight, but at the low end. The Pennzoil Platinum is supposed to be a bit higher within the specifications. But both should be fine to use. There might be some better, but those oils are harder to find.
There is a website that shows the specifications for many different weights and brands of oil, but I can't find it at the moment. Maybe somebody else would know, or when I find out I'll post it.