I'm skipping the Musketeers, mainly because counting Keldeo they're more of a quartet.
I honestly like Thundurus. It's got a cool design, it can learn a lot of great moves, and I actually think it's therian forme is pretty cool.
Well, the guy can calm down both Tornadus and Thundurus; plus in some scenarios you need both of the others to get him. I'd say he's the Trio Master.