Well they are all pretty amazing
But my personal favorites are Mew, Ho-Oh, Lugia, and Suicune
Mew- It can learn any move making it a really good pokemon, and in the movies and shows mew is such a fun outgoing pokemon. I wish all pokemon were like that. :)
Ho-Oh- It's just a beast, thats that
Lugia- Same as Ho-Oh
Suicune- I think its the best out of the legendary dogs