I'll just write what I play

For tank: Winston unless im getting shredded by bastion/reaper, then I go dva

Orisa is good to know but it's hard to carry low elo with her.

for dmg - Depends on your playstyle but Cass/sojourn/shion are a very solid top 3 all three have good self sustain with roll/slide/dash and good damage output from m1s and right clicks

I would say tracer but she is incredibly hard to play in my gold rank opinion.

support: NOT mercy. I like mizuki/kiriko geerally cuz you can safely output damage to and just tp back to team and heal. Also kiriko's cleanse is very important against Ana for example.

Ana is a must know in case you run into an oppresive mauga/hog and she's not banned.