I've been thinking of getting Vampire Survivor. Apparently it's basically crack cocaine and really addictive. I've watched someone play a round of it and I couldn't tell what was going on half of the time because there was so much going on on-screen, but at the same time it was very much a "mmmmmm numbers make brain happy." The guy I watched the video of compared it to a slot machine in terms of addictiveness. Plus, the game is only like 5 bucks.