Re: Best Marilyn Monroe collection?
Niagara and Bus Stop are both essential.
If you feel like dipping into DVDs, Monkey Business would be high on the list, but her role is a very small supporting one. Same goes for Clash By Night. Her part in Don't Bother to Knock is more significant and the film isn't too bad.
I also enjoy the light comedies Let's Make It Legal and We're Not Married- but again her part is small in both of what are ensemble pieces.
She also has a very famous cameo in All About Eve which is an all time classic Joe Mankiewicz film.
Out of anything, that is essential viewing IMO.