Pokemon with types\abilities that makes them "Bosses"

Title says it all:

Pokemon with unique typing are not an uncommon thing these days. Gen V intruduced many new type combiantions.

But what are the pokemon you take to a battle only because of specific combinations?
Or what are some pokemon you hate fighting against because of a type combination or an ability?

For me there are quite a few-
First is Emolga,this little guy is nuts (no pun intended).
Electric/Flying with a unique movepool and because of his typing-No ground move can hit him.

Second, pokemon with any kind of "absorb" ability are annoying.
"water absorb" or "lightning rod" for example.

And Snover and Abomasnow. They are ice/grass and are un-affected by their own hail.

I hate the design of Heatran but he is good at take Ice Beams, and Fire attacks,

Zebstrika was very useful when taking on Elesa's gym. Just switch into a volt switch and you're set for some serious sweeping.

In fact, the electric type really received some killer pokemon this generation. Between Galvantula, Zebstrika, and Eelektross, there are some powerful contenders with unique typings/abilities.

I'll have to say if it's one ability I hate, it's Prankster. However, any Pokémon with it immediately gets a higher attacking margin.

Alright, Eelektross is one damn hard Pokémon to finish; due to a combination of typing and ability. And it's not everyday you run across a mold breaker with Earthquake, is it?

Some Pokémon get their own level due to their typing, like Water/Dragon. Palkia is a one good example, he isn't challenged by dragons either, and still gets to use other moves to counter types of his liking.

PS - Sometimes Normal types get awesome too. They only have one weakness, so provided they aren't challenged by fighting, they also get to be impervious.

One Pokémon that truly stands out to me is Zoroark. It has a very unique ability, which enables it to transform itself into the last Pokémon in your party and trick the opponent. And since it has access to both Swords Dance and Nasty Plot, this will give you the chance to take advantage of the situation and use either of those moves to boost your Attack or Sp. Attack without being knocked out. If you're able to handle this Pokémon properly, then it'll be able to defeat your opponent's entire team single-handedly. d:

Darmanitan is also pretty special. It's the only Pokémon that has access to the Zen Mode ability. This ability changes Darmanitan's form whenever it's in low health, which allows it to take less damage from any moves and survive for a few additional turns.

I dislike fighting against Boldore because of Selfdestruct/Explosion and the improved Sturdy.

You're lucky then. Boldore doesn't learn Explosion until level 50+. But that's another thing. Pokémon with access to Sturdy can ruin the perfect sweeper out dry. And mostly they're rock types, meaning they can finish a lot of types.

I like the Wonder Skin ability, my Sigilyph in my Black version avoided so much damage with that ability it's ridiculous! It prevents your HP being lowered by sandstorms, hail and even burns and the poison status.
Also, the next Gen has a fossil Pokemon with the ability Freeze Skin/Refrigerate which changes all normal type moves to Ice type moves with STAB! That sounds really boss!