I don't know a percentage, but I can say this: Most of the women I ever fucked were friends with benefits that I fucked over and over again for months or years, and I always developed a very open sexual relationship with them where we'd talk really frankly about our sexual experiences and preferences. All of them said they loved fucking me to the point I was their favorite or second favorite fuck ever. If they preferred someone else, it was generally where emotions were involved with that other person.
Also had a few girls just once or twice who would say directly that they've "never been fucked like that" or that sort of thing.
Only three girls I fucked have been serious relationships for me, and two of those three said I was their best ever. The other just didn't talk about it. One of the two who said I was her best ever reminded me of it constantly lol. But I felt like she would be saying it whether or not it was true. In this context, I really trusted the FWBs opinions the most, since we would talk extensively and honestly about sex.
Size was a factor for pretty much all of them. A lot of the FWBs told me they just wanted to be fucked hard by a big cock sometimes, and I was their "big dick guy." They also appreciated me going down on them frequently, since most guys didn't do that often. And I think every one night stand mentioned my size positively.