based on what? PR, traffic, age?
Forget all that stuff. It's based on who needs it, and how difficult it is to find a similar domain. Look for a name where most unhyphenated (and some hyphenated) versions are not only taken, but being used by companies (not by domainers). Be sure there are plenty of companies in the same field for whom even a hyphenated domain would be a step up from what they are using now.
Nice search numbers are even better - but they are not the first thing you should look at. PR and age are mostly irrelevant.
Just put yourself in an end-users mind. Find some specific end users, and imagine you are working for them to find the best available domain for a reasonable price. First, you would look at the unhyphenated coms, nets and orgs. Are they all being used by competitors? Great for you. Is the .com priced by some domainer at some outrageous number? Good for you. Are they all held by domainers? Bad for you. (That suggests no end users are in the market for these names.)
At some point, a company will think to look at hyphenated domains if they can't acquire the first choices.
Longer term, all the new TLDs may lower the demand for hyphenated names - so don't go overboard on them. Also, you'll have to wait for buyers to come to you. If you go the outbound sales route, hyphenated names are a tough sell. So they are generally not good names for flipping.
edit: As pchip said, just stay away from more than one hyphen and hyphens that aren't .com.