North Berwick and Prestwick are right up there, no doubt. The different greens, blind shots, and quirky features make those holes a thrill. Can you imagine something more fun than bouncing it off the boundary wall into the fairway at Prestwick's opener? In golf, it might not be possible.
Leatherstocking is also near the top for fun. The cool short par fours, the mountain climb, the lakeside finish are all a thrill.
Inniscrone is a blast to play, for all its quirk and controversy.
Of my home courses, Deal is by far my favorite--hard to believe it's been a year since I last played it. The holes near the clubhouse, especially 2, 3, 15-17 (and also 6) are dynamite.
I thought Yeamans Hall (like other MRB layouts) was a ton of fun because of its wild features and its unique condition (which I consider to be perfect for my preferred type of golf).
Finally, another English layout, the Addington, was an absolute thriller, especially on the par fives.
In the end, what makes a course fun? For me, there has to be some quirk (like the Pit green or the first at Prestwick) and some element of man vs. nature (13 at Prestwick, 12 at Addington, and 18 at Leatherstocking fit the bill here). Ultimately, though, the most important element for FUN is SIMPLICITY. Inniscrone is my favorite course I've played this year, because it is a modern layout that is very simple. Hanse moved very little dirt, bunkered economically, and built greens with one or two dominant contours. This style of architecture doesn't get in the way of natural land features, and it creates strategic options that are timeless and ever frustrating. SIMPLICITY=FUN. Period.
Short par fours and firm and fast conditions don't hurt either.