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Vivillon is great. It's also really fun to use in Singles even if it's not actually very powerful; a Sleep Powder/Quiver Dance set with a Focus Sash and Bug Buzz and Hurricane for attacking is great, since Compound Eyes also saves the accuracy of Sleep Powder and Hurricane.
I personally use a Fancy Pattern one... basically just because I can, but honestly, most of its patterns are amazing and I'd settle for any of them. Monsoon, Elegant, Garden, Sun, Marine and Ocean are probably my other favorites.