How do I thank a professor for a great semester without sounding like a suck-up?

This semester, I had one of the best college classes I’ve ever had, and may ever have. It was on political analysis, and had great subject matter and an absolutely brilliant professor.

Now that the class is over and grades are out, the professor emailed me congratulating me on my performance in the class. I want to express how much I enjoyed the class, but I also don’t want to sound like a suck-up or, you know, anything inappropriate. Any help?