No option for Flopping Around so I didn’t vote. I’m usually on one side or another to avoid snoring, or on my back to cut loose, or on my belly if unaware, but I’m asleep then so how would I know?
I love being on my left side with my arm around Ms. P, but I can’t go to sleep that way. Sometimes we back up to each other, sleeping cheek to cheek. 
The majority of the time I sleep on my right side in a fetal position. I always think to myself that this is how I must have spent most of my time in my mother’s womb. I have my knees up, my ankles crossed and my arms/hands under my pillow/under my head. Or my arms crossed with the bottom one wrapped around to hold my upper shoulder. Once or twice throughout the night, I will switch over to my left side, the same position. Very rarely will I sleep on my back. I’m usually only on my back as I start to wake up in the morning. I remember sleeping on my stomach when I was a kid, but there is no way I can do that anymore. It’s been probably 30 years since I was able to sleep on my stomach. My neck hurts and my back feels like it’s swayed.
Sometimes I feel like my arms are in the way. I always think back to a David Brenner standup act in the 70s. He was complaining about how his arms were always in the way when he slept. He said he wished he could detach them at night and hang them on the wall. It was funny in the 70s!
Left side, although I wouldn’t say it’s my “favorite” position. It’s the one in which I snore the least. I don’t think I’ve slept on my stomach ever.
For decades I slept stretched out flat on my stomach. Then I broke my arm, requiring a cast from hand halfway up my upper arm, with the arm bent at the elbow. The only way to sleep then was curled up on my left side, with my left arm and cast over my head.
After the big and the subsequent little cast came off, sleeping on my left side was painful. So I switched to sleeping curled up on my right side, which is how I still sleep today.
I have a friend, that would always start sleeping on his back but would roll onto his side and then over again. One night he dreamed that someone was trying to strangle him. He started screaming in his sleep, thus waking up his wife, who found him wrestling around on his side of the bed, with his CPAP hose wound around his neck.
Next to Mrs Piper.