One of the reasons I like National so much is using Emerald Aisle service. The bus pulls up and you just pick the car you want. The full-size choices usually include Buick Century and Olds Intrigue, but very often there are Bonnevilles, LeSabres, Blazers, and even Sebring convertibles on the line. I really enjoy the new Cadillacs, too (yikes, I must be getting old!). My fave convertible by far are the Volvos that I've lucked into a few times on the West Coast.
I try to avoid anything with only 4 cylinders, most Buicks, and all Chevys except the Blazer. Ditto for any of the Ford premium cars like the Crown Vic or Marquis - they're just a bit too boaty for me.
Twice I've had the good luck to get a Jag, and that's enough to make you extend your trip... ;-)