Block Break - essential for fighting shielded enemies.
Countering Strike - essential for the entire rest of the game.
Countering Blast - very useful for the entire rest of the game.
Executioners Cleave - helped me tremendously because a good cleave can instantly remove a target from the battlefield or severely weaken one of the stronger enemies.
Every Atreus ability the instant it is available - his first abilities are so cheap it’s a no brainer to get them first. It makes him very effective in combat.