First scarlet, definitely
Wilbur - scarlet - boldtusk - gormek (care for overwriting - def) definitely make a strong off team, maybe pair with a healer in another color and you should get wins easier
Afterwards sonya, you need that dispel in many fights and she’s a solid 4*, eventually you get her costume which is awesome
Then maybe Mel-C, his costume is strong, still my best green healer
You really need a good amount of finished 4*, they are your main source for a long time, if you are not a big spender
I do play mono or 3-2 most of the time, and usually those teams consist of at least three 4*, sometimes even 4!
And I’m in diamond, those teams do have a power of around 3950-4200 and I beat 4,5k-4,6k regularly
And in the end, I recommend getting a good amount of 3* to compete in tourneys and events (if you are interested in that kind of content), gives more materials to upgrade your heroes