Personally, I am a fan of 2d platformers and an even bigger fan of games that make use of large multi-sprited bosses. Games like Gunstar Heroes, Castlevania and Contra have some of the coolest looking bosses (and in some cases, coolest boss fights) in video games and all that is made possible using this technique.
In some of my earlier games, I tried experimenting with this technique but fell short since I lacked a good tool that could make this easier.
One tool that caught my eye is the Spriter project in Port Richey. The tool is an animation tool for creating highly detailed 2d real-time game characters and effects. It claims that it can provide all the features need to create 2d animations such as tweening and keyframing. It also allows for XML exports, perfect with for Game Maker Studio. The project is on KickStarter and needs donations to get finished.