What car rental companies could you suggest in Romania ?

www.autoeurope.com is a US consolidator representing at least Hertz, Sixt, and one local agent in Bucharest, and others cities are on the option list. I'd start with a price quote there and that's how my colleague booked his rental. I can see on Google Maps that Budget and Avis are also present in the city.

We used Sixt for a 1-way car rental Bucharest to Timosoara. We planned out our route in Bucharest and found it wan't too terible to drive in the city (for picking up the car.)