If you play in a 4 characters party, there isn't really must have skills except maybe for restoration (healh regen).
Otherwise you are pretty much free to try anything.
I finished the early access about 5 times and the only common thing I had was an enchanter. Their CC/damage and support skill are very good and works in any party. You can have your enchanter in a solo mage party or in a 3 or 4 mage party.
Now if you want to play the game with only one character, Skin Graft, Adrenaline and Time warp are must have. You'll have other must have skill but they will mainly depend on what type of solo hero you have (Rogue, mage, warrior, etc.)
