So nowadays there's so many broken ninja floating around that I'd honestly say, save up for a good ninja.
Way back when I started, Shisuis, Kushimarus & Kushinas weren't a dime a dozen, now they're everywhere and the little nub, Tenten water blitz teams & Ay/Samui teams just don't cut it any more.

Basically you need a foundation to build from and that foundation starts at a Ninja, save up Coupons while spending time thinking up your ideal team in your head, something you want to be using for legit ever, I'd recommend Ninjas that wont fall out of the meta any time soon, so either bond Mystery Minato or Angel Konan- the easiest alternative to this would be to just grab Shisui & use Kurenai & Iruka, that's actually solid enough to get you through most stuff.

Someone here mentioned that the likely hood of your ninja becoming obsolete soon is a possibility- I'd honestly say he's incorrect, if you pick the correct Ninja he'll last a significantly long time- such as the ones I've mentioned, there's a lot of Ninja that are released constantly and the mass majority of them are actually rubbish, a prime example are the Edo Jinjurikis that all got released the past few months yet you see none of them, those are the kinds of Ninja you should avoid.

If you decide you don't want to grab a Ninja first, then your best bet would be Lightning main, you want to pull from the GNW treasure & hope you get lucky enough to pull the best F2P team, Kushimaru, Jinpachi & Sasori, or you could grab Roshi & run a cancer team that never gets old.

There's more I want to add but I have a really bad habit of over detailing my posts & then making them super long, I just want to add though that due to Sun & Moon, a large part of you power isn't heavily restricted to coupon saving any more, I think people are really undervaluing how big Sun/Moon is.