My top 10:
1). Hellsing OVA (yeah, I'm that predictable)
2). DRRR!! + Specials (LOADS of awesome adorable characters, urban legends alive, action, some romance, HIGHLY recommended)
3). Baccano! + Specials (from the authors of DRRR!!, also highly recommended)
4). Death Note
5). Ergo Proxy
6). Black Lagoon Series
7). Bakemonogatari
8). Tatakau Shisho - The Book of Bantorra
9). Shiki
10). Darker than BLACK - Kuro no Keiyakusha (too bad all it's sequels are really terrible

)
Also I'm currently watching Steins;Gate and I feel it will be at least seventh in the top once completed. Maybe even higher.