What was the shortest anime series you have ever watched?
I say Ouran High School Host Club
There are only 26 episodes..

I'm sure you've watched shorter. 13 episodes is how shows usually start off, and end up with 26 episodes if they're picked up for a full season.

Here are a few good ones to break your record.

Neko no Shuukai (http://www.youtube.com/watch?v=rQRQImMxvNU)
She and Her Cat (http://www.youtube.com/watch?v=wBD5D040D44)
Tooi Sekai [Other Worlds] (http://www.youtube.com/watch?v=9pWxRz-BsNo)

I think for most it would be FLCL.

Le Portrait de Petit Cossette. 3 episode OVA.

Heroic Legend of Arslan - I think it was 6 episodes, it could have used a full season but what's there is pretty nice.

FLCL, Magical Shopping Arcade Abenonashi, and Nurse Witch Komugi (which is pretty dang funny). All really good and short.

animation runner kuromi chan.
2 episodes.

FLCL
RG Veda
Can't think of any others at the moment. I think there's two more I've watched that were <12-13 episodes.

I agree with RG Veda and also the latest two Slayers. From what I can tell it was 26 episodes but half way thru they chopped them and gave them two different titles for the same series plot.

Yeah, pretty much any ova would do. Personal fave, The Darker than Black Gaiden OVA. It really took me by surprise by how much they put into 4 episodes in terms of story, action, and drama and I loved the subtle, yet profound emotional depth of it. The OVA totally beats out the second season imo

Mars of Destruction

bus gamer o_o now that actually was short if it wasn't an OVA I wonder. Only had 3 episodes I believe.

I don't think it really counts, but "Negadon: The Monster from Mars" is a single episode anime "series" on iTunes.

If that one counts, I'd say it was the shortest.

wtf, 26 episodes is the shortest? Is that the only series you've ever watched?

Ninja Cadets - 2 episodes
Blood Reign - 2 episodes
End of Evangelion - 2 episodes
Anime Runner Kuroumi - 2 episodes
Prefectural Earth Defense Force - 2 episodes
Puni Puni Poemi - 2 episodes
Time Bokan - 2 ep
Assemble Insert - 2ep
Tokyo Babylon - 2ep

and yes I recommend them all

EDIT: YEA!! 100 posts in four 1/2 years! W0ot!

She and Her Cat = 5 minutes 45 seconds long.
On Your Mark! = 6 minutes 30 seconds long.
Comedy = 10 minutes long.
Voices of a Distant Star = 27 minutes long.