I prefer a list... and not all "accidentally" do, but hey, maybe there's a reason life didn't evolve that way...
Metagross: Carnivorous Psychic Tank. (No more thought needed about that one: it just got hungry, and you are the menu)
(Mega) Gardevoir: Black Hole (Seriously, read up on the physics of that... bugger. And it makes me love her more! And she doesn't feel the pull of gravity.) Some idiot tries to kill her companion... too bad they didn't realize the consequences of the true power of love!
Dialga/Celebi: OP+commenters family lines never happened...?
Ditto: Identity crime made easy.
Porygon(2/Z): Cyber crime made easy.
Any higher-power Psychic: Mental takeover... The Pokemon trains YOU. Oh, and your mind is irreparable now, you're not coming back.
Voltorb/Electrode: Living, trainable, re-usable Bombs, a terrorist's dream.
Uxie/Azelf/Mesprit: Just read their Dex entries...
Jigglypuff: Puts idols out of business. Or sleeps them through their performance.