Possibly it's because I'm dark-haired, but I really don't get the appeal of Marilyn Monroe or Grace Kelly. Audrey Hepburn for me all the way! Those cheekbones, that vulnerability, her grace, her luminous complexion, her huge dark eyes - everything about her was perfect, as far as I'm concerned.
She was a good actor. Not right up there with (say) Meryl Streep or Judi Dench or Maggie Smith, but she was very good in the right role.
I love Breakfast at Tiffany's largely because of her - it's not a great film in truth judged as a whole.
The Nun's Story, however, is an excellent film - she got the Best Actress Oscar for her performance.
The film she was in with Cary Grant was Charade. Not a brilliant film but I'd watch it again just to see her.
On the other hand, I think she was miscast in My Fair Lady. (That was Julie Andrews' role on stage and she was miffed not to get the film role. She did, however, get Mary Poppins instead and as she got the Best Actress Oscar for that, I think she felt it all worked out OK in the end!)
Two pictures of her here.