Is it for the current stretch of the game with what's available before Terajuma or just an endgame goal for the team?
1) Ground. Mamoswine can be found in Anju's tower, extremely solid Ground/Ice, earth shattering Ice shards too.
For the future Krookodile with Intimidate (solid dark type too) and Flygon are both super solid ground attackers, and Gliscor if trained defensively is nigh impossible to remove without ice.
2) Dark. Houndoom is very very solid dark special attacker, also got Snarl to shut down special attackers and you should have it from Karrina and her quest. Unovan Zoroark is a bit of a glass cannon, especially until you get it's crest, but still very usable and fast, can find it in Mirage Woods on route 3. Speed Boost Sharpedo is a glass cannon but obviously can outspeed anything with protect.
For the future Hydreigon, Bisharp, Krookodile, Tyranitar, Weavile or Honchcrow all solid choices, hard to narrow it down.
3) Tank. Crested Phione is fairly tanky if you train it defensively and is something you would normally never use. Something like Slowbro is very hard to remove too. Gigalith is very fat and is a reliable Stealth rock setter if you want him to. Umbreon is notoriously fat and hard to remove and can Toxic stall until end of days. A ton of available options for the future too.