Lufthansa Miles&More offers 500 miles per rental with Hertz, Avis, Sixt and Europcar. If you're a LH member, that's quite good (no way to transfer these miles to a different FFP, though). With regard to actually posting the miles, Hertz was by far the most reliable and fastest, followed by Europcar, Avis (who tend to "forget") and Sixt (who "forget" even more).