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For me, it's Mewtwo. The best legendary Pokémon. I even train for extra SpA EVs and the game is so fun and more than willing to oblige.
One time I asked for SpA EVs and the wild Pokémon I was battling gave me three. I said, "Wow, three for free!" and the nice friendly wild Psyduck laughed and said, "I'm going to call you 3-for-free!".
Now the wild Pokémon greets me with "hey it's 3-for-free!" and ALWAYS give me three EVs. It's such a fun and cool atmosphere at my local training hotspot, I go there at least 3 times a week for levelling up, 1-2 times for Pokédex completion on the weekend, and maybe once for fun when I'm in a rush but want a great battle that is affordable, fast, and can match my daily EV gaining needs.
I even taught my Mewtwo a TM move, it's powerful! What a great legendary.