14,000 in a C-182 when I was flying jumpers, and I only stopped climbing because I didn't have oxygen with me. It was a cold winter day and I had the bare minimums for fuel, so I was still getting a 200 fpm climb out of the thing.
Also taken many piston singles (including my C-140!) and light twins to the 13,500 range on different occasions for various reasons.
Scheduled to take an SR-22 Turbo in to the flight levels next week also, weather permitting.