So… a dual-type challenge? If you want a balance of good offensive and defensive types, Steel is a must-have. It hits Fairies in X and gives you gems like Lucario and Aegislash. It resists a ton and has decently good coverage. For the other type, Water would be a good bet for overall coverage, access to Fly, Waterfall and Surf, and stuff like Gyarados, Ludicolo, and Greninja.