Celebi

Great League: 2 / 5

Celebi is a very unfortunate Pokemon that's basically invalidated by anything that can hit it Super Effectively. Its typing leaves it with a huge number of weaknesses, its stats are nothing to write home about, and it has a very shallow movepool. Psychic and Grass don't compliment each other very well, and Celebi doesn't have more useful options. On top of this, most Steel-types resist literally everything that Celebi can throw at them. All-in-all, Celebi will forever stand in the shadow of its fellow Mythic, Mew, and currently has no real chance at making waves in the Great League meta.

Ultra League: 2 / 5

Celebi carves out a bit of a niche as its CP is allowed to grow, finding itself able to counter threats such as Blaziken, Meganium, Toxicroak, and Venusaur. Its typing is still a severe issue, offering very few advantages and many weaknesses, but it's still capable of doing a good job where needed. Too bad other Pokemon can do the same job with more utility for the tier. 

Master League: 2 / 5

As before, Celebi has a few good matchups for the tier. It can take down Machamp, Swampert, and even Kyogre given the right movesets. Given some of the top threats in the tier and the fact that it has a relatively high max CP, it is capable of being a solid performer where needed, but it's still not exactly a huge force in the tier due to how shallow its niche is and how much it tends to overlap with other big-names such as Lugia and Mewtwo.