I basically cannot remember any rental with Sixt within the last 24 month without an upgrade. Usually reserve Golf-Category and the worst car I got so far was a VW Tiguan. The nicest upgrades this year have been so far:

FRA T1 : BMW M3 Convertible, Audi A8 4,2 TDI, BMW X5 3,0d and MB CL 500
TXL: Audi Q7 and BMW M3 Coupe
MUC: MB ML 320 CDI, BMW Z4 Roadster, Audi A5 Convertible

With Avis I got really lucky this summer at T1 in FRA. Customer booked a car for me (MB C-Class) and they gave me a Carrera 4S Convertible. It was only for 24 hours and return at the same station but nice upgrade.