I recently finished watching Code Geass, and nothing seems as good as that ;( I like anime with a really complex plot. Already seen Death Note by the way.
Stuff I've seen already:
Naruto/Naruto Shippuuden
Bleach
Death Note
Fullmetal Alchemist
Fruits Basket
will add when I remember what else xD
I started Fairy Tail but got bored...I've also started Guilty Crown (same writer as Geass), but it's not as good D: