My personal favorite has always been Porco Rosso - of all the Studio Ghibli films, it's the one I end up watching the most!
Mononoke Hime is a real classic, I liked it a lot more than Spirited Away and Howl's Moving Castle. All Ghibli films are good in their own way, but I guess everyone has their favorite themes and characters.
Another good one worth mentioning is Steamboy, although that's from a different studio (Sunrise, who also did Akira.)