It's really hard to pick any top #... a lot of them has many pros and cons... but I'll try... though I'd probably change my mind by tomorrow.

Top 5 All-Time:

The Melancholy of Suzumiya Haruhi
School Rumble
Gits SAC
Fullmetal panic
Elfen Lied

Top 5 Watching:

xxxHolic
Ergo Proxy
Ouran High School Host Club
Welcome to the NHK!
Utawarerumono