Here is a ranking for best mains to start with:

1) Fire (good early on in server, lacks a bit from lvl 40-55, then becomes good again. Also great in late game)

2) Wind main (only if you go for gaara)

3/4) Water and LM. LM is the best Main on days 1+2, but there is not much use for him in late game. Water is good from Day2 on forwards. Good in End-Game but you need good ninjas

5) Earth Main. NEVER EVER... EVER EVER EVER start with Earth Main. You will suffer until you reach level 60 and can change the main. He has nothing to offer early on. He is one of the best late game Mains in the game (probably the one you gonna swap to at lvl 60), but before he is level 55...he does absolutely NOTHING for you. He does not have his good support absilities yet and you don't have good enough ninjas to carry with him.

My favorite is fire main. He is good for Arena clear on day1/2. Gets a little bit chocked by Water main in Arena between lvl 40-55 (but that is ok) and is a staple late game. He is also excellent at clearing 9tails.

LM is pretty much always in top 2 of playabilitiy for new server...until you reach like level 60 (and can switch..) the problem is..after that you will barely use him. Not in the current meta anyways.

Water Main is good later on...but in the current meta inferior to Fire Main. But if you wanna sharkbomb arena while you can...

Win fell somewhat off. Still good early as well as late in the game.

You can change your main character later on (twice a day for free) so it doesnt much matter.
However...
Because you will most likely lack ninjas early on, my personal preference is Water main (shark bomb neurotoxin experimental team). Noted that Fire main might be better due to mirror ability and early ninjas are not immune to debuffs. So as a support, fire main is better.
So, I would start as Water main and unlock Fire main as 1st option (around lvl 40 or 50).

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you can switch later around lv 40, so it doesn' matter much