TRAPS: Give a perfect "10," and you'll seem too easy to please. Give anything less than a perfect 10, and he could press you to where you're being critical, and that road leads downhill for you.
BEST ANSWER: Once again, never be negative. The interviewer will only resent criticism coming from you.
This is a time to show your positivism. However, don't give a numerical rating. Simply praise whatever interview style he's been using. If he's been tough, say, "You have been thorough and tough-minded, the very qualities most needed to conduct a good interview."
If he's been methodical, say, "You have been very methodical and analytical, and I'm sure that approach results in excellent hires for your firm."
In other words, pay him a sincere compliment that he can believe because it's anchored in the behavior you've just seen.