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The best candidate for the role.
The best role for the candidate
The one that brings the most reward for the recuiter's efforts.

Now given meeting all three perceptions of best, or indeed any two of them is problematic, I personally have no problem with the recruiter going for door number three, after all I don't give a stuff if someone has an issue with me going for door number two...

That's honest, to claim anything else is dishonest. There again to say to a candidate or a customer you could do way better than this and not get paid would be most unexpected, though I agree most ethical.
Alan Sugar might personally agree as well, you'd still get to hear you're fired if it it was his recruitment company....

Developers churning out bug free code, an excellent allegory. Just as unrealistic.