I never saw Watership Down as a kid, I got it when I was like 22, and it still fucked with me. That is violent as fuck for a "kids" movie. I refused to watch Plague Dogs after how harsh Watership was, knowing that Plague Dogs was worse (and reading the synopsis for it). Another ++ to The Last Unicorn, the voice talent for that movie was superb, and the soundtrack by America really helps you get lost in the movie.
As far as surprised, the first Studio Ghibli film I saw was Princess Mononoke, which just happened to be on HBO one day. I watched the whole thing and was like, holy shit, is this that anime stuff that people talk about? Movie single handedly got me into anime and made me buy everything by Miyazaki. I'd have to say that Mononoke/Naussica are my favorites, but Spirited Away/Howl's are definitely up there.