Hawu

I do enjoy hots but I don't think steam will save it. The game will struggle to retain new players due to being unable to unlock heroes. If I play one game a day it would take months to unlock a single hero.

I'd order the top 2 as drone, queen. There is no specific single strong zero unit since it depends on the match up. Ling at number 1 for example if you make too many lings you lose the game at a high level of play. Their mostly just a stepping stone unit. While their good they get splashed to easily and are more of a meat shield protecting your gas units from taking the initial engage damage or flanks for positioning advantages.

Zergs advantage is drones they can produce more workers quicker.

Queens are the 2nd strongest due to inject.

You can skip any other unit on this list and win games but if you have poor injects or drone counts you won't do well in ranked.

Best upgrade for zerg hands down is Evo chamber as it applies to every match up where other upgrades are match up/composition based.