>>36805611The strongest story-mode mon is always Butterfree; it has Compoundeyes and gets powerful ranged/room-clearing moves early. Accuracy is always the problem with these games, and being able to Silver Wind an entire room successfully is really powerful.
Also, it has generally strong stats which last through story-mode and Group A IQ skills including Map Surveyor, Multitalent, PP Saver, and (for late-game grindan shenanigans, while you're training up its replacement) Pierce Hurler.