Best was a brand new Ford Mustang Convertible with barely 10 miles on the clock from an Intermediate class rental. I had it for 3 weeks too.
Worst was probably a high mileage, Toyota Matrix without any USB ports, the cigarette lighter port had a blown fuse and at speeds of over 50mph the car wanted to drift left so I had to hold the wheel slightly to the right... but I picked it up before they shut for the day, and when I noticed the issues it was too late.