One that really surprised me was Bring it On. I thought it was a really stupid movie to make into a musical (although it's a guilty pleasure movie), but I had tickets as part of my subscriber package. I tried to give them to a friend but she insisted I come, and I had so much fun.
I also love 42nd Street.
I thought Once and Kinky Boots were really enjoyable films, but I personally didn't enjoy either of the stage versions, so I guess that would go in the "unnecessary" category to me, although I know some people really enjoyed both of those. I hated Sister Act and Priscilla Queen of the Desert. (All of these in this category I only saw in Chicago and not on Broadway, to be fair.)
Is She Loves Me based on the movie or are they both based on the same book? (I haven't seen that yet but will this weekend and am so excited!) I guess there are a few that would fall into an ambiguous category. (Like the Color Purple, which was a movie first, but I guess they're both based on the book, rather than the musical being based on the movie.)