My favourite Ghibli's are probably
Totoro
Laputa castle in the Sky
Spirited away
Whisper of the Heart
and
Porco Russo
Although pretty much all of them are very good, and my top 5 change on a fairly regular basis (Totoro, Spirited and Porco all stay in there, but vary a lot)
I think I've got all the ones that have been released now, except Ocean Waves, and that's on hold because I've got the DVD's of the others, but I'm getting the Blu's as they are released (and there is no point in getting the DVD version now - although I have also got two copies of Spirited Away* and Grave on DVD).
I've lost count of the number of times I've seen some of the Ghibli's, Mononoke was one of my early Anime DVD's (I got the version with the book), and has been watched so many times whilst I've been working on PC's.
I've not watched it myself but I've heard it's a terribly sad film.
I haven't watched too many Studio Ghibli films (3 or 4 at last count) but Spirited Away is definitely well worth a watch if you've got even a passing interest in animation or more simply a good story.
Lets put it this way, it's pretty much the complete opposite of Totoro (and from memory was done at the same time, and sometimes shown as a double feature).
It's a box of tissues film, and one that although I bought a copy from the US about 6 years ago, then the UK release, I've only ever watched once.
It's pretty bleak.
*The lovely R4 limited release with the fold out cover of the bath house, and the UK release.