Depends on what style of anime you enjoy... Here goes my vote:
Full Metal Panic
Full Metal Alchemist
Black Lagoon (be sure to get season 2: "Second Barrage")
Ergo Proxy (can be a bit slow at times)
Naruto Shippuden (it's better than the straight 'Naruto')
Hellsing (and Hellsing Ultimate... and the three Hellsing OVA episodes)
I love anime... These are just a few off the top of my head. My collection must be in the upper range of 300GB at the moment between movies and series.