I've rented the 300, Genesis, VW CC, Maxima, Taurus, Lacrosse, and Volvo S60.
I like the 300 in Limited trim (haven't had the opportunity to drive a C), particularly its infotainment system. I also think it's a good looking car. It is a big, heavy car, and the V6 feels it at times. The new 8-speed helps.
I also like the Genesis. It feels like an expensive luxury cruiser -- but I'm not a fan of the blue radio display. I'm sure the next-gen Genesis will bring it up to par with the other premium vehicles.
Things I like on a "premium" rental (and granted, much of this is determined by how National equips their vehicles -- some are base models like the Maxima, while others are better trimmed, like the Taurus Limited):
- Leather interior
- Smart Key (keyless entry, keyless start)
- Infotainment LCD screen
- Auto-climate control
- V6 or a turbo-4
- Ample space for 4 adults with luggage (the S60 is out with its huge spare tire in the trunk)