Thinking up ideas is always a lot more fun than actually using them... Like, just an hour or so ago I got a rather brilliant idea for a level element............ in the final level of an episode........ which I'm still soooooooo far away from.........................................
But the best thing is, of course, the result. Nothing over playing what you created yourself a hundred times and actually LIKING it. Boo-yow.
Tileset and music are very important and very awesome if chosen well and fitting to each other, too, but I can't find anything fun in finding them, but rather in watching and hearing them harmonize when playing.


EDIT: Oh, and..
"adj. Informal

Enjoyable; amusing: “You're a real fun guy” (Margaret Truman). "

It's true, though, that there is no superlative of "fun".