You are an exceptional teacher, and I thoroughly enjoyed being in your class. I really appreciated and will always remember the interest you showed in my education.

With great thanks,

[you]

I thoroughly enjoyed being able to learn from you. You are a quality teacher and I will remember the things that you taught me fondly. The interest you showed in my education has encouraged me to continue pursuing my education further. You are a rare and exceptional teacher. Thank you for putting your time and effort into enriching the lives of students such as myself.

Sincerely,

Your Name

“Dr.Strongin, I thoroughly enjoyed being your student. You are a quality teacher, and I will remember you fondly and with great thanks and appreciation for the interest you showed in my education. Its wonderful to see that there are still exceptional teachers.”

If you use “fondly” it implies an inappropriate emotional issue. If you consider it amazing that there are still exceptional teachers, it sounds like you are taking a poke at Dr. Strongin’s colleagues. He or she might take issue with that.

However, it’s up to you to determine your own shade of meaning.