There are so many but the best for me are Spirited Away, Howl's Moving Castle, The Cat Returns, Whispers of the Heart (nothing really happens in this, tbh, but it's still pretty good, I actually really love it), My Neighbour Totoro, Ponyo, Porco Rosso, Princess Mononoke--I shall stop there because Id start mentioning every Studio Ghibli movie I've seen basically. xD I love a majority of their movies that I've seen, they're all pretty good.
Princess Kaguya was good too, I recently watched it but I was a bit disappointed in the movie, tbh... It was still good though and each to their own opinion, you'll have your own. xD
Studio ghibli is my life. quq I adore all of their films. There isn't one I dislike, not even a bit. But, lately, I got to borrow 'Whisper of the heart' from my friend and that is just a wonderfully delightful film. I also love 'The cat returns', 'Princess Mononoke' and 'My neighbour Totoro'. I have still yet to see films like 'Kiki's delivery service'. Can't wait for the next studio ghibli season on film4!
Edit: I can't believe I forgot Pom poko. Those adorable raccoons. ^^
OMG, I've been constantly singing it occasionally ever since I watched it. <3 Also, your mum sang it at school? Cool. It's a nice song. Why would she be annoyed? I would love to sing it in school (And might in the next music showcase ^^)
That's the same with me! xD Hmm, well, I guess when you grow up with something, it all turns out differently than listening to it for the first time in these days. To be honest, I'd probably dislike it as well. I kinda dislike a bunch of songs that I used to sing in school nowadays. xD (That'd be cool tho, good luck!)
My favorite has to be My Neighbor Totoro because it was my first from when I was very young and I get super nostalgic for it. But one of the best ones is probably Spirited Away or Howl's Moving Castle.