Well, I didn't stump you guys this week either!
"Saturday Night Fever" was the correct answer, though "Grease" was an excellent guess. I love watching dancing, though grace and coordination are not synonymous with my name. I remember being 8 months pregnant and going to see "SNF" and wishing I could disco. One of my daughter's nicknames when she would bounce to music as a toddler was "Disco Baby".
(And no, the fact that Tony Manero's dancing partner was names "Stephanie" had nothing to do with it! But it didn't hurt, either.)
There are all kinds of lists of best-selling albums, best-selling soundtrack albums, best-selling world-wide versus best-selling in the US, best-selling one-unit versus two-unit albums, etc.
Every list I find puts "Saturday Night Fever" above "Grease", though other soundtracks are emerging as better sellers. Of course, factors such as MP3 players, CDs versus vinyl, more movies, etc. have influenced these lists, but it's interesting to note that even 30 years after their releases, "SNF" and "Grease" still hold their own.
And Julie, although "Titanic" and "Dirty Dancing" sometimes top "Grease", "Saturday Night Fever" is always higher on the lists I've found.
New question on Saturday!