Pokemon I despise include:
Riolu - waste of Pokedex space and detracts from Lucario's awesomeness (and also ruins the chance for Lucario to be an official Legendary due to being a pre-evo!)
Phione - waste of Pokedex space and detracts from Manaphy's awesomeness. Even worse is that though it hatches from an egg produced by Manaphy, it can't even evolve into Manaphy! What's the point of the egg, then?!
Blissey - I hate facing this thing in Emerald's Battle Frontier; darn Double Team spammer! Normally, I support having no restrictions in battle (i.e., OHKO moves and Double Team are permitted), but Blissey is the one Pokemon that has made me understand the exact reason why there is a clause pertaining to Double Team in the first place, though I'm reluctant to implement said clause. A specially-oriented Pokemon versus a Double-Teaming, Softboiling Blissey is an absolute PAIN to endure. ARRRRRGH!!!
Jigglypuff - Wigglytuff is awesome. Igglybuff looks sort of creepy, but it doesn't annoy me too much. Jigglypuff, however, is freaking annoying in the Super Smash Brothers series, and took up what could have been a slot for Mewtwo's return in Brawl! That made me irate. How can a simple subpar jiggling puff last longer in the Smash series than a very powerful psychic cat that creates storms just by waving his hand?! It doesn't make sense?! To further worsen the situation, Jigglypuff's taunt is so freaking annoying! Hopefully, Mewtwo can return in SSB4 and axe Jiggly... (Keep in mind my hatred is only towards the SSB-featured Jigglypuff, not to regular wild Jigglypuff)
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Zorua is very lucky that it doesn't simulate Riolu's situation and fall into the above list. I actually like Zorua! Normally, I'd probably have hated Zorua for taking away Zoroark's potential for being a legendary (unless Generation V introduces evolving Legendaries which has yet to be seen, anyway), except for the fact that Zorua looks cute and cunning simultaneously. Furthermore, Zorua does not look like an obvious Zoroark pre-evo but rather fits its own design niche... Riolu, however, just looks like a lost Lucario with a severe growth deficiency/impediment.