Best audio upgrade for 05 Prius?

What would be your budget without breaking the bank ?.A decent receiver costs a 100$ with another 50-60$ installation fee.
Suggest you do it in an audio electronic shop , because in my experience they have more knowledge than Best Buy.
I recently upgraded my speakers to JL audio for about $350 or less while keeping the same factory receivers.
Apple CarPlay is good if you have a higher budget. but you get kenwood/JVC/pioneer from about $69 too.
If you also upgrade the speakers you will have a better experience.

 

IF you want just Bluetooth/handsfree calling, you can get Bluetooth handsfree kit for about less than $30 from amazon.
You plug that Bluetooth kit in your Aux port and you are there ready to stream music and receive calls.Sound quality is very so so...

 

any modern name brand head unit will sound better than the oem junk, so it almost doesn't matter what you buy. I would talk to crutchfield. they have everything you need, and they will help you determine which headunit has the features you want in your price range. but you will need a few other bits to make it work and retain your steering wheel button functions.