A big factor in determining a type's "spammability" comes down to specific moves or users. For instance, Dark is considered spammable not only because of it's good coverage but because of the effects and relative power of Knock Off; Flying, on the other hand, owes it's spammability almost exclusively to Talonflame and Mega Salamence. Ghost could be considered spammable but it generally isn't wise to spam a move that grants a free switch-in for Mega Kangaskhan, one of the most potent offensive threats in the meta, and Porygon2, one of the most potent defensive threats in the meta. Fairy is slightly more spammable than Ghost purely from a type-chart perspective, but falls short due to a lack of offensive users - the closest you can get to a Fairy-type sweeper is Unburden Slurpuff, which has issues of it's own, or Mega Gardevoir, which is hampered by it's physical frailty and it's 100 base Speed.
Of all the types, I'd answer with Dark: outside of Azumarill, Mawile and the occasional Conkeldurr or Hydreigon, Dark resists are not exactly common and the utility that Knock Off provides in particular makes it a tough move to switch in on no matter the damage output - the only Pokémon that don't particularly care about Knock Off are Mega Evolutions and Rocky Helmet users. Even outside of Knock Off, Hydreigon's Dark Pulse and the various Foul Play users are much more effective for Dark's outstanding neutral coverage. That's my opinion anyway - arguments can also be made for Flying, Dragon, Ice, Ground (especially with Mold Breaker)... almost any type. Gen 6 is a more level playing field than ever what with the increased viability of Poison and Steel as offensive types, so technically you could spam anything. Depends on team matchup and situational specifics really.
