This question can have many answers depending on what the question Is. Stat wise, arceus is the
Best legendary pokemon, but I think that deyoxis is the best with looks and usage

The best Pokemon, in my opinion at least, is Victini. Such amazing stats, astounding move pool, plus it's design is pretty swell!
Other than Victini, the only other legendary Pokemon I find good is Groudon. Same reasons with Victini, although I like how he is based of mythology, as I find it great to learn about mythology.

kirbmanboggle : But mew can be caught in wild in pokemon yellow in lvl 10 and it is also a weak pokemon. My vote will be the most for arceus as it was only pokemon which created a enter region. 

the best legendary Pokémon is #151 in the pokedex yea you know who I'm talking about mew everyone says mewtwo is stronger but mew defeated mewtwo yes mew had to kill himself to beat mewtwo so if you you want to get technical mewtwo and mew are equal so were talking about #151 and #150 in the pokedex because they are equal but one is evil and one is good mewtwo was the evil one the team rocket boss giovanni could not control mewtwo's power so he broke out and then after a couple of movie's mew kills himself to kill mewtwo proving they are equal in strength but not in heart so mew had to do the heroic thing and defeat mewtwo making mewtwo AND mew the best legendary pokemon in pokemon becuase back then in gen one being LEGENDARY meant something or you could say who gives a crap and say mewthree is the best legendary but really mew and mewtwo are

I have to say Dialga. Dat Dragon/Steel type though

Also, Dialga is a dragon that doesn't care much about the fairies nowadays. I love Dialga's design too...
And anyone that played the Mystery Dungeon Time/Darkness/Sky knows how dangerous a Roar of Time can be ^^

Darkrai because I don't know he just looks so boss.

mr.goblin : Welcome to vizzed. My favorite legendary is Victini because he can learn Reshirem's and Zekrom's moves.

Be easy to a newbie guys,

My favourite legendary pokemon is dialga, because it's looking cool and it's powerful to because it beats palkia right?
I Like The 3 Birds and Dogs to because they always do a gang bang

To me my favourite lengedary Pokémon is Reshiram.

I would have to say Dialga,dragon & steel seems to work together well for me. :3

Unions with their amazing attack HIDDEN POWAH!!

I would say Mew, with interesing attacks it's pretty useful.

Mewtwo is the best legendary pokemon ever.

Mine is Lugia. I love his design, and he counters Ho-Oh with his superior typing and moves. Awesome battle cry as well.

To me the best legendary Pokémon is Moltres

My Favourite legendary is Kyogre. I love kyogre and in fact I have made a lot of layouts of him.

He is just so epic. And he is also good stats wise. And How can you not love Kyogre?

Mewtwo is the most important thing EVER.

in all seriousness, mewtwo is my fave because he's the first pokemon I ever knew was a pokemon. mewtwo INTRODUCED me to pokemon. and that's why he's my favorite.

P.S.: Oh, and he's......some world ender Armageddon-y pokemon or....something like that.

ok my favorite legendary pokemon has to be garitena because he brings back so much nostalgia of my first pokemon game which was Pokemon platinum i have urged to find my game and get it back so i can play it again but i am haveing no luck finding my pokemon game and garitena is also my favorite because just like shaymin he has a sky form and a land form and i really like the texture put into garitena