We had a best experience with Sixt at Thesaloniki Airport.
Maybe its gonna be helpful for all to know, Sixt is ONLY rent a car agency in Thesaloniki which will exept Maestro debit card for deposit.
We actually booked another rent a car, wich I don't know how, bloked our credit card, so we was forced to search for another and Sixt was only one wich accepted maestro debit card.
Stuff was really helpful, wery friendly and I can say all the best. The whole process picking up and returning the car, was no longer then 5 minutes , and deposit for 2 days was only 100euro, wich was returned to my debit card when we brought the car back.