328i and 335d. I usually get the 335d if they have one available. Although I did spot a 5 series loaner driving around town recently. Haven't had another loaner since I saw the 5 so I can't confirm that thy also loan 5s on a regular basis.
carlsturm - My E60 M5 suffers from "RPM lag" - I'm not concerned with F10 turbo lag