Gah. ロマンジはよくないよ。
Another way to phrase this would be:
なん アニメーが いちばん すき ですか?
It has been a while but I think you just asked what anime do you like instead of favorite. Please correct me if I'm wrong as it's been 2 years since I last studied Japanese.
As for my answer: Anime is a bit tougher but Manga is definitely Nausicaa.
I'll make it by committee Ranma (nostalgia/my Naruto equivalent), Slayers/School Rumble (comedy), Mononoke Hime (flat out good), and Escaflowne (Wait the bad guy is who!?) I'm sure I'm forgetting some series that really should be the winner though