Wow. Lots of good choices here. I was going to say "When Harry Met Sally," "Sleepless in Seattle," and "Bridget Jones Diary," and "The Devil Wears Prada," but I see they've already been covered. So how about a couple that weren't such huge hits but are definitely worth checking out on DVD...
Music and Lyrics, with Hugh Grant and Drew Barrymore
I'll Be There, with Craig Ferguson (yes THAT Craig Ferguson) and Charlotte Church (yes THAT Charlotte Church)