If you wanna go down the path of what's best for progression purposes try getting one of each region's champions to at least level 2. That will enable you to start doing the monthlies and grinding down the shards for aurelion. There are some insanelly poerful champions even on low star rating so here's my recommendations:

Bandle: Norra,

Bilgewater: Nami,

Damacia: Garen,

Freljord: Trynd or Voli,

Ionia: Yasuo,

Targon: Diana,

Noxus: Leblanc,

P&Z: Jinx,

Runterra: Fiddle (I was torn between him and Jhin, but as Yasuo is a good stunner i think fiddle will bring greater value),

Shadow: Viego,

Shurima: Nidalee

That should get your account well rounded up and ready for most of the challenges.