Must haves don't really exist. There are several abilities that make the game much more enjoyable and help you find all the hidden goodies around the world.
Like kcnik stated above those are all good abilities. I would narrow it down to:
Pretty much needed - Teleport, Skin graft
Really useful - Adrenaline, mobility spells (spread my wings, phoenix dive, tactical retreat, cloak and dagger, netherswap), and buffs (haste, clear minded, Ice Armor).
Really fun - Source abilities. They are OP abilities with some fun mechanics, but aren't needed for any encounter.
But in general learn good combos for your builds. Enrage + snipe, Decaying touch + First aid. Impale + any fire source. Poison dart + any fire source. Rain + Electrocute. Rain + Contamination. Rain + Contamination + any fire source.