Definateley something classic like Shakespeare, but nothing that the agent would've heard a million times before. Try some of the lesser known monologues.
Then I would suggest something completely contrasting like a monologie from a really recent play, something perhaps that has been on at the Soho Theatre. You can look on the Soho Thetare's website for what's on:
http://www.sohotheatre.com/
Not sure where you live but the best place for scripts etc is Samuel French's bookstore in London. They do have website though that you can order from:
http://www.samuelfrench-london.co.uk/
Hope this helps?