Celebi

Celebi was the only Mythical Pokemon released in the Second Generation, only being obtainable by Events. When the second generation was first being released, Celebi was considered to be and called the "New Mew." Much like Marill (PikaBlue) was to Pikachu, Celebi had similar size and shape to Mew, and let's not forget it too was given a 100 Base for every stat category and was the final Pokemon in its given Pokedex.

But one thing Celebi can say that its Mythical Colleague Mew can't is that it was actually the first Event-Only to be obtainable in-game, after an Event Item was received at least. Celebi is also known as the Guardian of the Forest, and had a movie specifcally focused around it. Besides that, Celebi hasn't been seen much in the series. It isn't even possible to catch a Shiny Version of it, something that it and only about ten other Pokemon can say, which is pretty darn rare.

#251 - Celebi: As mentioned, Celebi was only obtainable in Pokemon Crystal in the Ilex Forest when you receive the GS Ball Event. Other than that, Celebi itself can only be obtained by Event.