Jirachi
I guess Jirachi isn't the worst mythical Pokemon, but it's far from my favorite mythical Pokemon and it's definitely not worthy of being the last mythical Pokemon left. Jirachi is a dual-type Steel/Psychic Pokemon introduced in the worst generation, Generation III. Because it resembles a star, Jirachi is said to be able to grant any wish written on the tags on its head when it wakes up once every 1000 years. Unlike Mew or Celebi which are heavily featured in the lore of their respective generations, everybody in Gen III could care less about Jirachi. Like other mythical Pokemon, you really have to get Jirachi through some kind of event. The first Jirachis were handed out to Japanese customers who pre-ordered tickets to the movie Jirachi: Wish Maker where the Pokemon was first introduced. Also because it's so heavily tied to Tanabata, there are usually distributions for Jirachi to acknowledge the holiday in game. Saving:
Infernape and Noivern