Jennifer Tilly in Bound or either of the Chucky movies in which she appears.
But I’m also taken by her other curves.
ROWR!
Greta Scacchi in The Player. I never tried so hard to develop X-ray vision in my life!
Yep, that one pretty much gets my Golden Nipple Award.
Colleen Camp as Yvette the maid in Clue and Franco’s distraction (:D) in The Gumball Rally.
Kitten Natividad in the Airplane! movies.
Kate Winslet, Thora Birtch, Christina Ricci
Absolutely anything with Jayne Mansfield in it.
Vanessa Angel. Spies Like Us. Was someone else in the movie?
Wasn’t she in *Presumed Innocent * with Harrison Ford? IIRC, you didn’t need x-ray vision in that one!
Ursula Andress in that James Bond flick…in that black and white bikini…totally hot!
Better yet, Vanessa Angel in Kingpin, when she gets the beers from the fridge. 
I’d have to say Maggie Gyllenhaal’s scene in Secretary was about the sexiest I’ve ever seen, but the OP said
BOSOM
and in that case, I gotta give it to Kathy Shower’s hot-oil rubbing by firelight scene in “the Further Adventures of Tennessee Buck.” If you put Gyllenhaal’s sexy acting in Shower’s body, she would implode into a black hole of sexiness and devour the entire planet. Which would be a bad thing … I guess …
I am just posting here to denounce the distressing dearth of links in this thread. Shame on all of you!
Here you go. Don’t hurt yourself.
My personal Top (or is that Topless?) 10:
Kelly Preston in Mischief
Betsy Russell in Private School
Molly Ringwald in Malicious
Lea Thompson in All the Right Moves
Alyssa Milano in Embrace of the Vampire
Athena Massey in Undercover
Catherine McCormack in Dangerous Beauty
Maruschka Detmers in The Mambo Kings
Shannon Whirry in Body of Influence
Jennifer Ehle in The Camomile Lawn
With honorable mention going to…
The entire female cast of The Bounty
Hottest movie bosom: The Iron Giant, when he goes into full-attack mode. That green energy must be nuclear…“hot” in both the temperature and the radioactive senses of the word.
What?
Rosanna Arquette in “The Wrong Man.” No contest. It’s a good thing the movie overall is so sleep-inducing, or I would have torn something.
Kelly McGillis in Witness. Were I Harrison Ford, I’d have raised something besides an Amish barn. 
Not to be too obscure, but how about “obscure forgotten actress with breathtaking assets in that forest scene in Friday the 13th Part V (at least until the predictably stupid murder ruins it)” as a nominee? That was a favorite when I was younger.
And I get to be the first to name Jamie Lee Curtis in Trading Spaces. Woot!
And don’t forget her sunbathing scene in S.O.B. Yowzer!
If I may sidetrack into TV land, this reminded me of Shirley Jones in the call-girl outfit at the end of the skunk episode.