The spark is SUCH a good term for it. I've had it be a scene, a scenario, a line of dialogue, but it's the push to make you write it.

the scene that gets the most comments or the line that gets the most cites is almost always the spark. That is SO cool - you obviously put a lot into it and people pick up on that!

I immediately write that scene first, and then start filling in the blanks of how it got there

WOW. I very rarely do that! I think I mostly need to write fic in chronology or IDK I guess I get worried I'll lose motivation to write the rest if I get my favourite part out first or something. Oh that is SO interesting.