Celebi

Celebi was the last pokemon to appear on the Pokedex in the Johto region and being Pokemon #251. Celebi is a Time Travelling Pokemon, allowing anyone to go back into the past. 
Ontop of that, Celebi can also heal the worlds forests.
==In the games==
Celebi may have a glaring 7 weaknesses due to the typing of Grass/Psychic, but it also sports 6 resistances. Celebi resides in the OU tier in Pokemon despite the 7 weaknesses including a x2 weakness to Pursuit and Ice Beam, However being able to resist Lightning and Ground.
Celebi is considered either one of the best Baton Passers in the game (Just above the Eeveelution) and one of the Best SubSeeders. 
The fact Celebi can diverse away from many recommended movesets proves how good this little pokemon really is. And a solid 100 base stat in all her stats make her an extremely versatile pokemon allowing to go physical, special, wall, tank or else wise.
Celebi was such a powerhouse, that in the GSC environment, Celebi was banned. In ADV, Celebi was borderline Uber, but in DPP she is now considered a solid OU pokemon.
==In the Anime==
Celebi makes three appearences in the Anime. First in the movie Pokemon 4Ever along with another legendary pokemon Suicune. 
The second appearence was when Ash and Gang went to save the Celebi who got trapped within the forest to try and heal it.
The third appearence was in a special when a young Ritchie managed to travel back in time due to Celebi.
The original saga in the anime with the GS Ball was originally meant to contain a Celebi. This was dropped in favour for the movie.
==Other Appearences==
Celebi appears in 1 in every 251 Pokeballs in Super Smash Bros. Melee. She gives you bouns points aswell as her trophy.
Celebi also appears in Brawl in 1 in 493 Pokeballs in Super Smash Bros. Brawl. She will drop a trophy for those who let her free.
==Members on Celebi==
* Kuja loves this Pokemon and named it Serebii. When first trying out NetBattle, Rey suggested this Pokemon to him, and ever since then Celebi has been his favourite pokemon of all time. (Above Suicune and Moltres.) Celebi is his main experiment, choosing different movesets each time to see which one works.
* Rey - Rey thinks Celebi is GAAAAYYY. And is probably Satan.