In Alexander Mackendrick's weird, bitter, depressing Tony Curtis comedy Don't Make Waves, Cardinale is so completely outclassed by Sharon Tate that Andrew Sarris remarked that it was "the biggest Hollywood cheesecake robbery since Betty Grable displaced Alice Faye in Tin Pan Alley."
If she's the only woman in the movie, as she is in Westerns like Once Upon a Time In the West and
It's not that she isn't attractive, just that I don't find she lives up to the sex-symbol buildup she gets in most of her movies. I have my issues with Marilyn Monroe, but it's impossible for anyone to steal a scene or a movie from her. Ditto Sophia Loren, Bardot, and to some extent even Lollobrigida (who made mostly mediocre movies, but managed to dominate them). But some attractive actresses, no matter how aggressively they were sold as sex symbols, come off as no more intriguing than other pretty women in the same film, and sometimes less.
Sometimes I think Anita Ekberg is in a similar category for me -- though that's much more controversial, because unlike Cardinale, she did achieve legitimate sex-symbol status for a while. But again, I find that unlike a Monroe or a Loren, it's very easy for her to be outshone in the movie by the woman who is not supposed to be the sexpot, whether it's Aimée again in La Dolce Vita, or Pat Crowley in Hollywood Or Bust (who somehow manages to be sexier than Ekberg even though most of the film and promotion was about how sexy Anita Ekberg is), or Edie Adams in Call Me Bwana.
I don't know if anyone else has their own examples of an actor who got the sex-symbol buildup but is rarely the sexiest person in the movie. And obviously, this applies to male actors just as much as female.