Naruto (original, haven't started Shippuden yet) is probably my favorite OST too. Few music themes can make my blood boil as much as "The Raising Fighting Spirit" or "Strong and Strike", while "Sadness and Sorrow" is just as breathtaking, but on the other side of the coin, managing to be both uplifting and heartbreaking at the same time... I can't imagine the series without it, so I know I'm going to be inevitably dissapointed with Shippuden's music... I should check more of Toshio Masuda's work...
The ones that closely follow it for me are the ones from Rurouni Kenshin (series) and Evangelion (original). Both of them also unforgettable. And after these, the ones from the original Saint Seiya and Captain Tsubasa series would round up my top 5.
Since we're posting examples here, I'm going to leave one of my favorite tracks from Saint Seiya, which appears in my favorite anime movie, "Shinku no Shounen Densetsu"/"The Legend of Crimson Youth":
http://www.youtube.com/watch?v=wifO6K7fLM4 (Abel's Theme)
