What is a piece of writing I should make a choral song for?

I honestly have no preference currently, it's just a matter of what others think I should do.

Something old enough that it's in the public domain and then you can compose and promote it without having to worry about getting permission from the copyright holder?  Or write your own poem (if you're a decent writer).  

If it's a sacred text that opens up millions of church choirs that might want to sing it.  If it's seasonal, secular, and of an easy difficulty level that will appeal to millions of school choir programs that are looking for things for the "spring concert" or the "holiday concert."  Pick your target choir type and then design the piece around them.  No sense pairing a babyish text with difficult music, or an inaccessibly dense intellectual text with music for beginning sight readers. 

You probably stand the best chance getting programmed by a school or church, as opposed to a community choir.  Millions of churches need 2 or more pieces, 52 times a year, so they are happy to spare the budget by getting a free score by an unknown composer off the internet.  If it doesn't go well, who cares.  They'll make up for it with the music next week.  Millions of schools need pieces that meet very specific difficulty and vocal range requirements, but still sound nice, and they have budget issues too.  Community choirs are generally better funded for the purchase of music because they can just make it a membership requirement that everyone writes a check to cover the cost of their own music at the beginning of each concert season, and they perform much less music in a year, so they are less interested in unknown composers.  They can't risk programming something that doesn't help them sell tickets.  Churches and schools have captive audiences of parents and congregation.  Their chief concern is affordability and appropriateness to the sermon or season, so if you find a niche to fill, they'll program you.  (: