Probably Tsutaaja.
But usually, it depends on the kinds of Pokémon in the Pokédex. For example, in Diamond and Pearl, there were barely any fire types in the Sinnoh Dex; if you wanted a fire type without picking Chimchar, you had to have a Rapidash. So I picked Chimchar in Diamond. Now, you can already tell there's going to be an abundance of water types in Isshu, as with every new region, so Mijumaru is out of the picture. Hihidaruma looks like a cool fire type, anyway, so that'll probably be my fire type. So naturally, I'm getting Tsutaaja.