Your company may have changed because Avis is relatively generous on kickbacks to the company on corporate accounts, but is far less rewarding to the median traveler than, say, Hertz.

If you have an Amex Plat you're entitled to Preferred Select (the level you can earn as a frequent renter). Some frequent renters have been known to be given Presidents Club (minimum 2 car class upgrade, guaranteed availability) but it's rare -- usually just a certain # given out w/ large corporate contracts, and also given to holders of an Amex Centurion card. Chairman's Club is much harder to obtain still.