1. Poison/Dark – I love this type combination because it counters Poison's weakness to Psychic-type and includes all the added benefits of Dark-type such as Prankster immunity, which is rampant in the VGC, and the ability to take down Psychic and Ghost types. This is why Drapion is one of my all-time favorites.
2. Psychic/Dark – It's pretty much where opposites come together. This is a great combination for reasons similar to Poison/Dark, but it also makes the Psychic-type more of an offensive threat and protects it from Ghost-types offensively and somewhat defensively. This is why I love Malamar so much.
3. Fairy/Dark – Again, opposites come together and these two types can help each other out when paired, as they both do well covering each other's weaknesses, especially offensively.
4. Fire/Grass – I haven't gotten the chance to use this type yet, but I love weather and terrain. A Pokémon with this type-combination would be great with harsh sunlight and Grassy Terrain up at the same time.
5. Ice/Ground – This type-combination covers pretty much the vast majority of Pokémon in the game. If you love offense like me, you can't go wrong.
6. Normal/Psychic – Allows Psychic-types to completely be immune to their Ghost-type weaknesses as a trade for neutral damage against Fighting, which doesn't matter because Psychic wrecks Fighting anyway.
7. Fairy/Steel – This type-combination is op because you combine one of the best offensive types in the game, which is Fairy, with arguably the best defensive type in Steel, and you have a great combination here.