Oh, where to begin? There really is nothing like being in a live audience to see a musical or play, but if you lack the time/transport/funds, youtube isn't such a terrible substitute.
The list of what I have seen is pretty long, but I'll list what I can remember.
Seen versions in both Stage and Film:
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Singin' in the Rain
The Producers
Sound of Music
Chicago
Phantom of the Opera
Cats
Annie
Les Miserables (I preferred the 10th and 25th anniversary concerts to the 2012 movie, but it is breathtaking on stage)
Wizard of Oz
Grease
Rocky Horror Picture Show
Little Shop of Horrors
West Side Story
Jesus Christ Superstar
Joseph and the Amazing Technicolor Dreamcoat
Film Only
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Love Never Dies
Rock of Ages
Into the Woods (stage production and movie versions; hm, now I think about it, there might be a local production this year)
Jersey Boys
La La Land
Stage Only:
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Wicked
Matilda (the movie is not the musical; the musical is more true to the spirit of the book. Comedian Tim Minchin wrote the music/lyrics)
Avenue Q
Spamalot
The Lion King
We Will Rock You
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I plan on seeing Book of Mormon and Aladdin in a few months' time. I've been lucky enough to see some of these shows more than once, but they've been spread out over the last 20 years.
I am aware that musicals/theatre are not everybody's preferred entertainment, but there are so many different shows out there that I want to believe there's something for everybody. I'm curious about Groundhog Day, which is Tim Minchin's next production after Matilda. I've heard at least one song from it, and that hit me right in the feels. However, I have few memories of the movie, and am lukewarm towards it. Whether I see it will depend on the price and timing.