There are two occasions I would call the best ones:

1. My first time, when my fwb surprised me with it.
She ate my ass and fingered me for the first time, noticed that I like it a lot and told me to wait a moment. She got around this room divider and came back wearing a strap on an matching lingerie.

2. A threesome.
Same fwb wanted to explore her bi / lesbian side in a threesome with me. We found a pretty lesbian who was bit bicurious too. First I mostly just watched them doing it for a long time, then the new girl told me she would love to watch my gf pegging me. So we did that. They both got very dominant and I loved it.