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Are we talking lore wise or competitive wise? If the former, definitely Tao. I really like how they used to be this all powerful dragon probably even stronger than Arceus. I also feel Kyurem has the most tragic backstory of any legendary. 2 brothers argued about truths and ideals, so The Original Dragon split itself to satisfy both brothers. Unfortunately, Kyurem became nothing but an empty shell that can't even feel emotion. No joy, no sorrow, no nothing. He still waits to this day for the hero who will finally complete him.
Now if I have one criticism for this trio, am I the only one who's not a fan of Reshiram/White Kyurem's designs? Part of it comes from the fact that her (Yes I think Reshiram looks like a female, at least compared to the oher 2) wings are attached to their arms. I thought it looked stupid when YZard had it, and I think no different for Reshiram.