2D Shadow of the Colossus. I really enjoy games like this that essentially strip the fat off and say ok the fun part of the video game are the bosses so thats all we're giving you. A lot of the challenge of these games is usually either trying to puzzle out the bosses weakness or being good enough at the game to defeat them, but this game does a fantastic job of combining them with having your arrow one shot every boss with very strict timing windows and having to figure out how to get them to expose their weak point in the first place.