I'd live my life as a Shiverian if I could. Or poke my way to victory.
They tried to win us over with a T-Rex in the reveal trailer, but the best transformation in Mario Odyssey by far is the Shiverian seal. On the one hand it's not grossly overpowered like the Pokio transformation, but on the other...you get to be fat and bounce around like a ball, and I don't think it gets much better than that.
Playing as one really reminds me of playing as a Goron in LoZ Majora's Mask, where you assumed the form of a bulky, slow moving creature, only to quickly realize that it's actually a magically powered race car in disguise.