Such a subjective topic. Perhaps the most fun I've had on a course had nothing to do with the course itself. It was a nondescript resort course in mid-Michigan and the weather played a factor. One hole it was cold and snowy and the next sunny and more tolerable. Hot chocolate was the drink of the day and the company was excellent.
I also had a great time playing The Loop (Red and Black). The fun was due to the myriad options off the tee and especially around the greens. I played terrible but had a blast. I also found Pasatiempo a very fun course. An easy walk and a fun finishing hole. Not to mention an eagle on 9.