I have usually found Hetz to be one of the cheapest. Pepecars might be cheaper , but its like flying with ryanair, you pay for everything and its more hassle than its worth.
One thing I would recommend is getting excess insurance at home before you go, The car hire places offer this to you at the last moment and the prices are sky high, you dont need to take it, but if you want peace of mind so you dont have to pay an excess fee if something happens, get it at home 1st.