It's a classic question: If you could meet anyone, living or dead, in all of history, who would it be?
It reveals some things about your values, your interests, your character. It can be fun rhetoric.
Pare it down to your career. If you could work with anyone, anywhere, doing anything you love to do, what would you choose?
Or more importantly, who would you choose NOT to work with?
It seems a little simple at first, but if you know your value, are confident in your abilities and can really bring the best solution to the client, nothing should stand in your way towards success.
Alternately, you should know who isn't your customer as well as you know what foods you would never want to even try (canned spinach, anyone?)
Success grows slowly but surely. Along the way, experiences mold you into what you're meant to be. One tripping point is the test of your values and how your want your reputation to grow.
Sometimes who you work with determines who will work with you!
This is not about judging so much as being aware of how you may be judged. I joke that I "don't judge anyone's money." We focus on what we're hired to do. We focus on the service - there's no duplicity, secondary motive, alternate agenda.
Of course, there are limits to who we'll work with - anything that is in direct opposition to core values is avoided, but even that is very pragmatic. You can't be everything to all people (so settle for the majority!)
When presented an opportunity, take a beat and think of how your involvement will be perceived and how the resultant work will reflect on you. Don't let present desperation determine your future.