Are u talkin about in the game or in the cards?
Game: Fav - Castform
Strongest - Deoxys!!!
Cards: Fav - Rayquaza d (delts species)
Strongest - Metagross ex (hidden legends)
In the game, Kyogre is so broken, it is ridiculous. It is definately not to be played with in competitive matches.
Chances are, Ho-oh or Mew were meant to be supreme.
My favorite legendaries are Zapdos, Raikou, Suicune, Rayquaza, Ho-oh, and Latios. I simply can't decide which one out of those is my single most favorite.
Mew, on all of the pokédex pages he's said to be the strongest. Besides, he has the DNA of all non-legendary Pokémons. This would mean he can use transform on his own DNA and transform in a pokémon which his opponent has weakness against. Or he can use every attack =). Mewtwo is a stupid copy, he can't even learn every attack!!!
Ho-oh is awesome and all but its a contest between Metwo and Deoxys. Deoxys can dominate stat wise in any field but Hp(Shuckle sucks so we ignore him) but metwos pretty tough to beat. Id still go with Deoxys thanks tot he move pool from all the games and the fact his stats are obscenly high. And Lev 30 starts no bad.