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You can use whatever you like competitively.
I like using legendaries because that way I get to find out their strengths and weaknesses better. I wouldn't easily come to the conclusion that Heatproof Bronzong can beat Xerneas if I didn't use Xerneas.
If you don't like a certain pokemon or think it's too powerful, use them for a little while. Take note of what beats them so you can adjust your favorite pokemon accordingly or use some new ones of your own.
