Off the top of my head when it comes to the genres I read the most:
-The Girl with the Dragon Tattoo. The nordic thriller genre was highly prolific well before it, at least since the 1960s, but Larsson popularised it to a wider non-Nordic public by introducing cartoonish, exaggerated, violent, movie-like elements that contrasted with the tradition both of Nordic thrillers and of crime novels in general, and formed the basis for many successful writers that came afterwards.
-Battle Royale. Pulp, extreme and theatrical, it is a landmark of dystopian fiction that IMHO deserves to be in the same realm as 1984 and Lord of the Flies (not necessarily in prose quality, but part of it is because it’s not in English originally and the translation is bad) and is perhaps the single most important work of the whole “killing game” sub-genre.
You may no like it but 50 Shades of Grey made erotic fiction a mainstream phenomenon for a while. Also, I have no idea which book started the Holocaust-themed historical fiction books, but it must have been hugely influential because that niche exploded, at least in Portugal ans probably the rest of Europe.
The Boy In The Striped Pyjamas (2006) may have the been the catalyst for that particular genre (Holocaust, not erotic fiction).
That would be my guess as well.