So here are the top five spells I use in the Open Beta. Please note I discounted spells like ray of frost and firebolt and that cleric cantrip that can do damage.
5) Greese-If you put this strategically at a chokepoint it slows melee units down something awful. It is always difficult terrain and it may cause them to slip and fall and end up prone, wasting their movement for the rest of the turn. Fantsastic spell. I give it to everyone I can. Plus, not even concentration!
4) Shillelagh: Turns your pure caster druids into melee beasts. Because you use your wisdom to hit and damage, Suddenly your 8 strength druid is hitting like a fighter. Best part about the spell is that your quarterstaff does as much damage 1 handed as it noramlly does one handed, so you can use a nice shield with it.
3) Thunderwave: This spell is best when Gale is an evocation wizard, as it then ignores your friends. Just knocks enemies back, off cliffs, and it doesn't do bad damage. Particularly fun in the tower fight when you can knock the robots down like 4 stories and give him tons of falling damage.
2) Spike Growth: Oh wow, it's like greese, but over a wider area and it does damage as they move. Put it in a chokepoint and enemies LITERALLY kill themselves trying to get to you. Only at number 2 slot because only Druids and Rangers get it.
1) Cloud of daggers: Wow, this is just a phenomenal low level spell Just the best. Put it at a chokepoint and enemies take boatlods of damage as they walk to you. In the bridge outside the goblin fort, I have had just, goblins run into it and die by the dozens over my gameplays. Plus, if you put it on someone, it does damage to them twice. Then with thunderwave, you can throw them back INTO it. So good. Has won me more fights than I can count.
Honorable Mention: Counterspell
While phenominal, you don't get it until level 5 (max level for the game) so I didn't feel right putting it in this list for the early access game.