Rock/Dragon
Rock/Flying
Grass/Fairy
I think that Fire and Flying resistances are much more important to have than Ground, Fighting and Dark, as those can be patched easily by either a teammate like Tomohawk or by simply outspeeding the most relevant Ground types, Lando-T and Zygarde, as have easily explotable weaknesses, while the former makes you have a harder time switching into important threats like Pyroak, Tomohawk and Heatran. Besides, while the lack of a Water resistance may be a unappealing, you still take reduced damage from them under both Sun and Sand (on the special side at least).