Unpopular opinion, but Manaphy and Phione. I've always found it cool that Manaphy is the only mythical Pokemon that can breed and have an offspring that's related to it, but cannot evolve into it. I've also used Phione multiple times throughout playthroughs of a Pokemon game, and I really enjoy using it. I sometimes wish Phione wasn't disputed so much as a mythical Pokemon because I'm so fond of it...
Also, I went out of my way to get Manaphy in my first playthrough of Pokemon Diamond, so, yeah.
also I accidentally voted Arceus on the poll because of the title being "best mythical" WHOOPS