First Generation: Zubat and Golbat
Annoying freaking Pokemon you encounter EVERYTIME in caves, and they are used by evil teams such as Rocket, Magma, Aqua, and Galactic. They also confuse your Pokemon.
I hate these things!
Second Generation: Ledyba, Ledian, Marill, Azumarill, Hoppip, Skiploom, Jumpluff, Sunkern, Sunflora, Unown, Wobbuffet, Dunsparce, Shuckle, and Delibird.
'Nuff said.
Third Generation: Azurill, and Wynaut.
Prevolutions of the above. 'Nuff said.
Fourth Generation: Bronzor, and Bronzong
Same as Zubat family, except they have the same type as Beldum, Metang, and Metagross.
Fifth Generation: Roggenrola, Boldore, Gigalith, Timburr, GURRRRRRDURRRRRR (Sorry, just couldn't resist that meme), Conceldurr, Trubbish, and Garbodor.
Roggenrola, Boldore, and Gigalith - Same as Zubat family, and Bronzor family. The differences is that they don't confuse you, but have a sturdy ability, making them annoying to defeat.
Timburr, Gurdurr, and Conceldurr - Stupidest English Pokemon names. They're easy to make fun of because of the "Herp" and "Derp" meme. I feel sorry for those Pokemon being named like that.
Trubbish, and Garbodor - A Pokemon made out of Garbage? Who came up with this monstrosity?