This is pretty subjective, but I'm going to try to come up with a good list of not conventionally beautiful women that have turned me on over the years. There are going to be some repeats for sure, but also some originals.
Ellen Barkin - Definitely an unconventional looking but sexy AF Jewess who did a lot of nice sex scenes and nudity in her career. Her sex scene in Bad Company was a stand out for me:
Brittany Murphy - She was the plain Jane to pretty girls Alicia Silverstone and Stacey Dash in Clueless, but she had a sexiness all her own. R.I.P.
Dominique Swain - Few would call her beautiful, but she was sexy AF to me. Her career fizzled out after a hot start, unfortunately.
Juliette Lewis - Was great at playing young, sexy characters in the 90s. I wouldn't call her beautiful, though. But the type of girl you'd love to rail.
Maggie Gyllenhaal - Far from beautiful, but has shown her sexiness in movies like Secretary or TV shows like The Deuce.
Chloe Sevigny - From getting plowed while out cold in Kids, to sucking Vincent Gallo's cock on camera in The Brown Bunny, to shows like Big Love and Bloodline, she's always managed to bring the sexy despite not having cover girl looks.
Selma Blair - Ever since her breakout role in Cruel Intentions she's been sexy AF.