I can't believe that this thread has got to 4 pages and no-one has said "South Park: The Movie". So I will!
Mrs Nerd is a big musical fan so, while I hadn't seen many musicals before we started going out, I've seen plenty over the past 5 years.
Favourites for me: Chicago (film or stage, they're very similar), Little Shop of Horrors (film version) and Avenue Q (stage). Along with South Park, of course.
Least favourite: Moulin Rouge and Rocky Horror Picture Show (I think you had to be there at the time for RHPS, nowadays it just seems like a bunch of students acting badly and trying to shock).
The first 15 minutes of the Tenacious D movie is almost the best musical ever. The film heads downhill pretty quickly, though.