Got lucky this weekend and managed to snag an S5 for a couple of days. The classification was UXAR (premium elite) so same as Audi A6, Lincoln Continental and Infiniti Q70.

Setting aside the Miata, the S5 is by far the best handling car I've ever rented. It grips so well that you can't really come close to its handling limits on public roads. It was so much fun to drive in the mountains.
The other highlight of the car was the exhaust: in dynamic (aka sport) mode, it crackles and pops when up shifting or letting off the gas at above 4000 rpm.
The only downside I would say is the ride - it's a bit stiff on bad pavement.

In terms of best handling cars in the National fleet, I would put the Miata and S5 at the top. The next tier would be the ATS, CTS, and Mercedes CLA.