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I used to run Eviolite Gloom as a joke pokemon back in the day and it was actually pretty decent. Giga Drain is pretty good when you don't have recovery items and moves like toxic, moonlight, stun spore and sludge bomb were all pretty decent. One of my favorite sets because it was hilarious when it worked.
Another option is Golbat who is pretty fun, basically any mon that got an evolution in later gens is pretty strong with it.
Another one worth mentioning is Clefairy: Magic Guard, Cosmic Power, Moonlight and Stored Power (though clefable does this better still) and in doubles friend guard is pretty decent.
You probably won't be winning tournaments with these but it's really funny taking down randos with them.