Hi there Logophile,
First of all .. because i think, you need Monologues for Auditions - ill suggest you a very good book concerning Auditions and how you behave at Auditions, how you prepare and so on:
http://images.amazon.com/images/P/05...CLZZZZZZZ_.jpg
Because you often have to Cold Read at Auditons, means you get a Sheet of Paper and than you have to play it nearly out of nothing, without any Preparation and Work on the Role.
As for Monologues. Learn whatever your Heart burns for. For example, when i started Acting once the Time came up my teacher said to me: "Well, i think its time for you to work on your first Role, choose one until next Lesson". So i inquired about Roles and Monologues, and a few days later i found Stanley Kowalski. I thought, wow .. this is a cool Character, i like him .. although he is the absolute Opposite to my Personality, but that made me Courious to work on this Role - it affected me somehow, it aroused my curiosity, in addition Stanley was a Anti-Hero to me and that pleased me .. so i bought the book - and started to work on it.
For the Future, it would be great - if you construct and prepare a whole Bandwith of Monologues for yourself, about 15 or 20. Classical and Modern ones - Both are important and Useful. So if anyone anywhere at what time ever asks you for a Monologue - you will be prepared to show your Abilites and your Skills. In Addition, who ever asks you for, he/she will be able to choose out from your Archive of 20 Monologues. That shows you are Multifarious - and an good Actor should always be Multifarious.