The ideal user to test is someone from your "target audience." They should match the most important characteristics of your user persona. (You did develop a user persona, didn't you?).

Sometimes, however, your target audience may be unavailable or too expensive to bring in (say, for example, doctors).

In this case, try for a good match, rather than a literal match. Instead of doctors, men and women in age range of 30-50 who are professionals. Then again, maybe medical students or office managers from a busy practice would fit the bill.

When it comes down to it, however, testing ANYONE is better than testing no one.