A few years ago, I attended an invite-only private sex party in Orange County CA as the guest of my roommate's friend. I drove to his place in Long Beach and rode with him to the party. Along the way, as a favor to the party host, we stopped to pick up a sexy 18 year old Latino, who had just graduated from high school two weeks earlier; the kid had recently done a solo JO scene for a porn movie produced by the party host. The party was just getting going when we arrived, so I wandered off by myself to explore the scene. I walked into a dimly-lighted room with nothing in it except for a canvas sling in a portable frame and two guys in their late 20s who were making out intensely. As I'm heading out of the room after taking a closer look at the sling, the two guys grabbed my arm and pulled me into their embracing and kissing. Without evening saying anything, they stripped off my clothes, put me in the sling and took turns rimming and fucking my hole. After each guy had cum in my hole, they hung around for the rest of the party watching me get fucked, felching the cum out of my hole and whispering in my ear the running tally of how many different cocks and cum loads I'd taken.
At one point I noticed that the 18 year old Latino kid had been watching from the back of the room for quite a while. I motioned for him to cover towards me, which he did just as the guy fucking me pulled out, so the kid opened his fly, took out his rock hard 8 inch uncut cock and then proceeded to jackhammer my hole for a few minutes until he shot his load deep in my hole. He smiled at me as he pulled out and I thanked him.
When the party started winding down, the two guys kissed me goodbye and left the room, but they soon returned with a studly black guy in his early 40s. Apparently the guy had a huge cock and loved to fuck white ass, so they told him about my ass and brought him to check me out. Unfortunately he had been just cum after fucking for a couple of hours, so he wasn't able to get fully hard, but even just partially erect his cock was at least 11 x 8, which he was able to put in me since my hole had already been opened up by 18 different cocks (and 13 loads).
Not sure if this is THE most slutty thing I've done, but it's definitely in the top 10.