Right seat, Shrike Commander 500S. It was a charter flight from Lizard Island to Cairns, Australia, with about 8 passengers on board. Shortly after takeoff I told the pilot that I was an instrument-rated private pilot and much to my surprise he gave me the plane and told me to fly us back to Cairns! It was about a 45-minute flight through benign cumulus buildups; probably 50/50 VFR/IMC, hand flown with an unfamiliar panel. He did a great job guiding me along, though, and he even let me land the aircraft! Somehow managed to slick it in real nice and got a fantastic ovation from the passengers. I'll never forget it. Here's a pic of the captain and I afterwards:
Close second would be riding jumpseat in a Varig 737 from Sao Paolo, Brazil to Rio de Janeiro.