I would recommend Sixt for a car hire. It's on the expensive side but you get what you pay for. They are quick to respond to emails and phone calls and are helpful with requests and questions. The staff are friendly and helpful and there's no hidden charges or fees as you get with a lot of car hire companies. Everything went smoothly for us, we even accidentally crashed into another car (minor incident, not even a dent) and they were kind and helpful, they weren't worried at all they just let us get on with our trip and with the full insurance we didn't have to pay anything extra at all.
The rest of this review is most helpful for under 25 drivers.
We had to choose Sixt because they were one of the only two companies who would hire to under 25's, the other one was a super dodgy one so we went with the more expensive one. Had to fork out almost €2000 for 17 days of car hire which was insanity to us but the experience was worth it in the end.
If you do hire with Sixt as an under 25 driver, I have some advice. Bring your documents with you, and something you will need is proof that you've had your licence for more than 4 years. I didn't know that so I was freaking out looking through my emails in the sixt office, it would have been nice to know prior. Other than the massive extra fees even though I was literally less than a month off the 25 mark, it turned out fine in the end.