Depends. Usually anything that uses a confusing move or all-hit move in PMD, because it is SOOOOOO FRUSTRATING slowly dying because you can't move out of a hallway, or 3 firkin Porygon Zs are spamming Discharge.
Aside from the one that everyone knows I hate, I'll just say my second least favorite.
Lucario (Not the one I'm talking about) is an awesome pokemon with an awesome typing. He's the closest we get, other than that smugly pony from gen 5, to a special fighting type. Thanks to this, he's one of the few that are able to use Vacuum Wave to a good use. I love him quite a lot. He's my team leader on Black 2.
So after a while, I heard that Lucario got a counterpart in gen 5. I was excited! I loved lucario in gen 4, so it would be awesome to see another one! I was in love with it...
Until I found out what the counterpart was.
I looked at it, and thought "Really? THIS is what they made for Gen 5's lucario? I am so disappointed."
It was the pokemon that nearly made me hate Gen 5, the one that made me hate the dark type, and the one that was my least favorite for a very long time.
Zoroark is quite frankly the opposite of what I love about lucario. Being dark type, Zoroark can't get any decent STAB going, since dark type moves are weaker than any other type. He's weak to the most common type in competitive play, and while his ability is decent, TEAM PREVIEW EXISTS, D*** IT! While I do fall for it constantly (Trust me, I do a lot, which is another reason why I hate it so much), If you're smart enough, you'll almost never fall for it! He's also really ugly! Zoroark is quite easily my least favorite gen 5 pokemon, and my second least favorite pokemon overall.
TL;DR: I love lucario, Got excited for a new lucario, lost all excitment when I found out what it was, Hate zoroark.
Official Qwilfish hating and Art loving Shadow Zangoose of the X board! ZANGOOSE for Smash 4! Also waiting on Zangoose's megalution, ZANSHRED!