I don't think we have many legendaries available on R/G/Y to make an interesting poll... I counted 5 on total: Moltres, Zapdos, Articuno, Mew and Mewtwo.
IMO, the coolest legendary in this generation was... Mewtwo. It is so cool that though being "two" it comes first than the Pokemon it was originated from, Mew. Mew is also cool, but I prefer Mewtwo for any reason.