Good question because it makes you realize how "fun" is only a tiny part of the overall gaming experience (for better or worse). Off the top of my head, feelings like tension and excitement, wonder, frustration, addiction are other qualities and reasons to why the overall gaming experience is satisfying (can you come up with other good words to describe the qualities of a satisfying gaming experience?).
When I think "pure fun" I think "Arcade gameplay" rather than the typical HD game. In a modern gaming world that means Nintendo, and unfortunately I'm outside the world of Nintendo.
So for me "fun" is old coin-op Arcade games with the Mame emulator on PC.
Mainly Bomb Jack.
But out of HD games I think the highest proportion of fun moments I got was from Red Dead Redemption and Vanquish
