That is a tough request, but in no particular order (it would take far to long for me to try an order them, hehe)
Negima (The original series, newer one just isn't as good IMO, not to say it is bad, but just not as good)
The Melancholy of Haruhi Suzumiya
Welcome to the NHK
I tried hard to trim it down to five, but just couldn't do it.
This is a small sampling of what I've actually watched, but these definitely stand out for me.
I'm currently watching Bleach on Hulu, but they are slow to get episodes and I'm not really a fond of fan sub versions of any Anime. Aside from that I am on hold for volumes 3 and up of Black Lagoon through Netflix, which is how I've seen most of my Anime Series