I'm always happy to battle any Pokemon! Except maybe...

Clair's Kingdra from Gold/Silver/SoulSilver/HeartGold. This piece of $**t wipes out my team every time. Not even a Jynx from Ice Path or a Dewgong can stop it. It's freaking unstoppable.

shedinja!
i never have any pokemon with anything super-effectice against bug or ghost types whenever i fight this thing.

Ugh. Spiritomb. Walls my Normal Monotype team horribly. I should probably train an odor sleuther or something but ehh.

In the Battle Tower/Castle/Arcade it was always Garchomp.

We all know how powerful it is. It's the 1 thing that makes the Battle Frontier completeable but at the same time makes it almost impossible when you're up against one.

You'd only ever have narrow escapes from one at the best of times.

I feared with Koga's Venomoth or Agatha's Gengar because of their Supersonic or Hypnosis respectively.
I really hate those "wise" trainer. They really know that they need to put
their enemy into sleep/confuse/paralyze/poison first for easier to win (like a normal person).

For Venomoth: Koga
For Gengar : Agatha

If i have no fire pokemon then.... Shedinja!

Recently, I found myself fearing those pokemon who keeps on spamming double team...

In Platinum, Glameows and Purruglys. Those things are the bane of a Nuzlocker, I tell ya'.

The Dewgong in Gen 1's Elite 4. Thing would use Rest all the time. I didn't fear it, it was just annoying!

Emolga. There so annoying, always using double team

Collectively, my friend's Sandstorm team. Or it was, until I put together a Quagsire to help me crush them, and a Torterra to help him help me crush them. For us the game is like an arms race but with bioweapons.