2009 Nissan Frontier -- A solid truck that drove like a car. I used it to haul stuff and it did a commendable job. It had what may have been the worst factory radio I've ever heard in my entire life. The cargo bed extender was clever though, and the back seat was genuinely spacious.
2008 Saturn Aura -- This was the XR with the 3.6.
I put 10,000 miles on this rental and loved it. I took it across the country, to the Bonneville Salt Flats, to the Redwood forest in California... everywhere! By the time I turned it in the brake rotors were toast, though it was likely due to my abuse. Epsilon is an excellent platform. Except for some very minor interior foibles, GM did a fantastic job.
2006 Cadillac DTS -- Outstanding cruiser, though fuel economy (21mpg) wasn't quite as good as the Seville (23mpg). Excellent sound system, beautiful interior. Enterprise took an hour though to clean the car and get it ready for me while I waited. I called and specifically reserved a DTS. Took it to Nashville.
2009 Cadillac CTS -- Rented from Budget. A completely hassle-free experience. Rented it for the entire weekend and took it to Chicago. My legs were too short to comfortably reach the pedals.

I guess that means I won't be owning a second-gen CTS, ever.
2007 Chrysler PT Cruiser -- The car drove fine. I didn't understand what people complained about, however, this one smelled like diapers and wet dog so I took it back. The guy at the Enterprise counter in Tampa was kind of a d!ck.
2007 Kia Optima -- Average car that drove like mush. What a difference from the new Optima.
International Box Truck -- From Enterprise. Easy rental experience, interesting spring-loaded seat.
2009 Ford E-Series -- Econoline van from Enterprise. Good customer service, surprisingly nice and capable van.
2007 Mazda 6 -- From Hertz in Tampa. I MISS THIS CAR. The suspension was a bit too soft but the steering and chassis were amazing. I got hooked on it.
2007 Mitsubishi Galant -- From Budget. Took it from Washington to Iowa and back. A completely forgettable car. It was reasonable roomy, reasonably efficient, reasonably styled, had reasonable handling, and... yeah. It was between that and a Dodge Neon so I'm glad I picked the Mitsu, but there are times where I completely forget that I had ever driven one. Most unremarkable vehicle ever made. Budget was awesome though.
Overall, Budget is my favorite rental service. Enterprise is great for commercial stuff. Hertz was excellent but they don't always have the best deals.