There are conflicting reports of what the world's shortest poem is, with Guiness claiming that a four-legged "m" is the shortest, but since that isn't a letter, or even remotely pronounceable, I'm going to ignore it.
Muhammad Ali came up with what I believe to be the shortest poem, with his magnificent:
Me?
Oui!
when in France as a young man*.
The oft-quoted:
I?
Why?
doesn't do it for me. Written by the ever-famous Anon - god, I cannot believe that guy's bibliography, why is there not a forum dedicated to him at this site? - I'm not convinced it's fair.
I think the problem cuts to the root of poetry in that a definition will be subjective.
Please give me your thoughts.
* I get a lot of laughs out of a NZ rugby coach who taught his players: Me? We! as a mantra, utterly missing and completely changing Ali's brilliance. Also, it's a letter shorter and could reasonably qualify under the same rule that Ali's does.