They each counter each other, it's like pokemon, fire

Agree with that option. This game was made to counter each class by their respective weak element. If you think about wind can blind or remove that huge buffs fron lighting teams while spamming clones to protect from the leawt life attack that it has. Fire will debuff the wind since they don't have any good removals and he is good at aoe attacks and support too. Water is mostly a support that heals back dmg and removing debuffs but it is also a cc class and has dmg potential through poisoning tai. Earth is the tank in the game that can make the fight longer through stacking up shields and buffing the defense or go for multiple attacks and buff for early attacks to do a lot of dmg at the beginning. Ps how that earth counterd water is still unkown xD. Lighting is best at dealing huge solo dmg and stopping chases and synerzizes well with other swordsman

Currently, unless you are a super whale, the only viable mains are azure fang and midnight blade. Breeze dancer and scarlet blaze to work nicely need some hard to get ninjas and crimson fist currently works well only with one specific lineup.

There isn't such as "Best Element", every element works good with certain ninjas. And every element has its own specialty. Fire main is good at controls, water main is good at healing, lightmain is good at speed, wind main is good at clones and earth main is good for shields.

Each one has it's benefits,

Light is for high damage/blitz, Earth is for tankiness, Fire is for disabling/ignite, Water for healing/durability, Wind is for clones/buffing ninja like Naruto and Gaara, who are common in game.
I would suggest playing Fire until you hit the 80 - 90s, in which then you should try out Water/Lightning depending on the ninjas you have.

It depends alot on your play style really, and even so, each element can be viable to take on a few types (e.g. Fire- CC, or AOE).
Maybe earth... because he rocks. Sorry, could not resist.