TRAPS: Give a perfect 10, and you'll seem too simple to please. Give anything less than a perfect 10, and he could press you as to where you're being significant, 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 the time to show your positivism.
Though, don't give a numerical rating. Simply praise whatever interview style he's been using.
If he's been tough, say you have been by and tough-minded, the very qualities required to conduct a good interview.
If he's been systematic, say, you have been very methodical and analytical, and I'm sure that approach results in excellent hires for your firm.