I notice a lot of people are suggesting poison/ground, but I think the positives of poison/ground are all found on poison/fighting with added bonuses. Poison/fighting keeps the fighting/bug/rock/poison and adds dark and grass resistances. But without the water and ice weaknesses, making it a better wall for fighting types in general and generally less vulnerable. It adds a flying weakness, and I know all 3 cap projects have a flying weakness, but the fact is, flying is just not a common attacking type. It also compounds the psychic weakness, but if I had to pick a type to be 4x weak to, psychic is probably the one I would pick. Nobody has psychic unless they get stab so it's hardly an unpredictable attack. Just something to think about if you are allured by the tempting resistances of poison/ground.