I love this thread!
Worst would probably be the time I was doing it doggy style with my ex, and without asking me or warning me, he just went straight into anal sex. It hurt so much and I told him to stop but he didn't, at least not for a few seconds until I pushed him off and scrambled across the bed! We'd never done any sort of anal play before or even talked about it, so it was a shock and painful without any foreplay or lube.
Second worst would be similar to yours OP - A friend and I decided to have sex with each other, just once, because we were going through dry spells. He had a tiny penis, not just in length but in girth, it was seriously thin! That wouldn't normally matter, but he didn't make any effort during foreplay to make up for it. When we were doing it I couldn't feel a thing, and within 30 seconds or so he just pulled out because he was 'done'. 30 seconds of awkward, silent, boring sex, for nothing. So we never did that again!
Best time is very hard to choose, because since I met my OH 5 years ago we've had so much amazing sex. With him I've explored with sex toys, bondage, spanking, and all sorts of fantasies really. Even just vanilla sex is incredible with him I'd probably have to say the most memorable was when we were staying in a really nice hotel in Cambodia, and washing each other in the walk-in shower. One thing led to another, we performed oral sex on each other, then he had me pinned to the wall, lifted me up and penetrated me... doesn't sound like much but it was so sexy. After a while we moved on to the bedroom, and that was the first time he made me orgasm through just penetration while he was on top. It was probably one of the most intense orgasms I've had. Not the most adventurous sex we've had, but the most pleasurable.