I'd like to say it was with my wife, but really, it was with the woman I dated before her. This woman had just separated from her psychotic, cokehead ex-husband; we met because of mutual friends. Although she was very attractive to me (tall and curvy), she was quiet and shy and I probably would never have made a move.
But one night, her friend asked me to walk this woman to her car, because she didn't feel safe. I did, and invited her back to my place; we ended up fooling around, which was great.
She invited me to go hiking with her the next day, which was also great, and then I kind of invited myself back to her new apartment. For the next 4 days, we did almost nothing but have sex, eat (a little) and sleep (a little). It was passionate, wild, mild, sensual — and everything in-between.
We were together for the next four years, and I wanted to spend the rest of my life with her. Unfortunately, I didn't want to have kids and she really did. So that was that. But the sex we had, especially the "lost weekend", was without a doubt the best sex I've ever had.