1. Magicarp (lets see you take a splash and live to tell the tale)
2. Sunkern (An absorb off of base 30 special attack, definitely a game breaker)
3. Capterpie (That string shot tackle combo, it's deadly man)
4. Luvdisc (look at those eyes, pure evil)
5. Unknown (a highly unpredictable special sweeper)
Safari: Dragon | Gabite, Noibat, Sliggoo
Pokémon X | FC: 4871-3955-6532 | Name: Brandon
Be humble for you are made of earth, be noble for you are made of stars.
In-game, based on base stats:
2 Dialga, Giratina,
Ho-Oh, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Zekrom,
11 Groudon, Kyorge
15 Pseudo-Legendaries, Chibi Legendaries, Darkrai, Deoxys
In the anime:
1 Arceus-You should know.
5 Lugia-Sexy Aeroblast that destroys the crap out of everything.
5 Mewtwo-Erm, look at the movies?
5 Rayquaza-Settling them down.
It's difficult to objectively rank their power from a myth/lore perspective because the games and other source materials are pretty inconsistent in terms of what legendaries are capable of.
From a purely battling perspective, that task becomes much easier.
2. Arceus (Normal)