Even though I hate the Hoenn, I have to second Breloom. Breloom is one of those Pokemon that can come from left field and completely ruin one's day.
Take for example the fact that it can learn spore and up until recently Spore worked against all types and put everything to sleep with 100% accuracy.
Then couple it with wake up slap, so it not only gets STAB off the move, but it can consistently do huge amounts of damage to your opponents who have no defense outside of abilities that keep them awake (or being the Grass type in lieu of generation 6).
Now, with it's ability Poison heal it can become quite the nuisance when you give it a Toxic Orb to hold. So not only does it heal itself, it's Poisoned so you can't burn it, Paralyze it, or Freeze it, rendering several combat styles void. The only way to truly combat Poison Heal is to negate it's ability, which can be voided by switching out.
Breloom can also learn Facade. With no ill effects to speak of, Facade doubles its attack power from 70 to a whopping 140. Only a handful of moves deal 140 damage without charging/recharge, have more than 100% accuracy, or have more than 10PP or isn't a signature move. Facade is a guaranteed 140 Power/20PP move when combined with Poison Heal.
Aside from that, Breloom's great move pool can combat nearly any type. Breloom can be pretty fragile, but it makes up for that by being a glass cannon that often explodes in your opponent's face and not yours.