Unfezant - It's a shame I could never use it the way I used Staraptor on my team. It just wasn't powerful enough for my tastes.
Zebstrika - Its visual appealing is amazing, but it was way too fragile.
Archen line - I really dislike drawback-only abilities, even for flavor reasons. What's the point of halving stats when below half health when you can't even kill anything when you're at full health to begin with? (That's just my experience.)
I'd say something about the Pans and Simis, but personally I think those were made to get you past Striaton City. Of course, I'm stubborn for thinking this, since I never used them afterward, but still...
And in the same vein as the Archen line, Regigigas. I know it's not a fifth generation Pokémon, but I just hate its Slow Start ability.