best audio speakers and headunit for 08 prius?

I recommend alpine tweeters. assuming you don't have the jbl. the alpines clip right into the tweeter triangle and have a very clear sound. I had them in my '04 and I could hear the stick hit the cymbals like they were in the car.
I also used visaton front woofers and Dayton audio 7" rear woofers. these woofers are not car speakers, they are for home audio but worked fantastic in my car. sounded better than the jbl system using a kenwood head unit.

 

Best is relative to cost. Tell the audience how much you want to spend and you will get better results on your request.

 

if you can diy, that should be enough. the Dayton Audio RS180-4 7" Reference Woofer 4 Ohm from parts express.com are about $50 each, and they work damn good. my brother is an audiophile and he was impressed when he heard my car. they won't fit in the front door because they'd hit the windows but they fit good in the back doors. get a 2DIN touch screen to replace the junk oem headunit, a metra axxess to keep the steering wheel button functions, and a scosche install kit, and your good to go. but solder your connections, and use shrink tubing. do not do the idiot install by twisting wires together and taping them. and t-taps suck too. don't use them. I would call crutchfield for the head unit and install kit, alpine tweeters from whoever has the best price and parts express for woofers.