For me personally, it is Mew. Although there are many other fabulous Pokemon like Lugia or Typhlosion. Another one is Charzard. For me Its ho-oh the three legendary dogs charizard and typhlosion :)
Go back to New Bark Town. Use surf on the pool to the east of town. Once you're on that trail, keep going. There's lots of ace trainers on the way, so pack accordingly. Eventually, you come to a...
When you get to Kanto, you have to go to Lavender town Radio tower and get the EXPN card from an old man inside. Then if you tune your radio to the top, you can use the Pokemon flute channel in Kanto...