In this brawl, everyone runs minion heavy decks, often with cheap, 1-health minions on the board from the get-go so Argent Squire and Voidwalker are invaluable. That's also why Knife Jugger and Mortal Coil are decent keeps.
Flame Imp is just huge value for 1 mana so always keep that.
Only keep Abusive Sergeant if you've got Argent Squire in hand.
Harvest Golem is a good follow up to Knife Jugger on a clear board, it's not a must-keep but it's a lot of value for 3 mana.
The most important starting cards are Voidwalker and Flame Imp but Argent Squire and Mortal Coil are both decent keeps, together they allow you to remove a 2 health minion and draw a card which, at the cost of the Divine Shield, is strong in the early game.
Acidic Swamp Ooze is a good keep against Hunter for the Eaglehorn Bow which will inevitably be dropped on turn 3 – Warrior might not run the Fiery War Axe since the nerf but…