well... for starters, ghibli restored my faith in anime, as a lot of these manga adaptations were beging to put me off... so i love them all because they have a reall sence of culture and fantacy behind them.
i'd say Princess Mononoke, probably purly on the basis that i aor wolves. the soundtrack is stunning (as is generally the case with ghibli) but i did prefer the one to spirited away.
i own thoses two and grave of fireflies, whichi'm still waiting for a free moment to watch all th way through *has very litle time, but has owned it for 3 months already!*
from what i've een of it sofar, it's very different to the other two, but i think i might actuall grow to prefer it... i just need a couple of uninterupted hours to watch it properly