Jirachi, his ability is the best one for battles.
No.3 - Phione . Now most peeps hate this thing because it is exactly like manaphy but I love the idea of a Rare mythical coming from an egg also it’s shiny is nice. No.2 - Melmetal . Love how he was a new mythical introduced for Pokemon go and let’s go also his evolution process is all about fusing plenty of meltans according to the anime. Also like phione , neat shiny. No.1 - Jirachi . I just frigging love this mythical so much!!! it is cute and looks like a star and the Pokédex fact is so cool since if you write your one true wish on one of its tags when it’s sleeping it will wake up and grant that wish then sleep for another 3000 years. And like all the others I love it’s shiny.
I'll have to say Darkrai. Most mythical's are created for the sake of a new generation and don't have much backstory to them. This one had a good personality, it's own games, backstories, movie, and overall are my favorite Pokemon of all time, hands down.