Diablo IV - I don't trust the devs but I enjoy the genre.
Before that I pre-ordered Warhammer 40k Boltgun. I just knew from gameplay I was going to buy it day 1 and I found a good deal for it prior to it releasing.
Before that it was RE4 Remake. I knew I was there day 1, so how could I pass up $43?
I usually only preorder if I know I'm going to buy a game regardless AND I can find it for a good price before it releases.