Well, my collection currently holds over 144.000 pics total and keeps growing so that's a pretty hard question.

I mostly gather stuff with less common themes.
Especially my bondage and futanari folders are pretty well filled.
I'll save anything I like in my mixed collection though so it's not like I wouldn't save an image of a half-naked ship or an embarrassed fox, it's more that I feel like these specific images don't speak to me in a way that'd make me want to have them.