Druid spellcasting isent so good for doing direct damage (some exceptions aside) but are very good at support casting, debuffing or buffing.
Lvl 1 gems:
Faerie fire (specially early on) can give your entire party advantage on enemies who fail their save against it. Its a dex save if memory serves but against groups generally at least a few fail their saves. Given the effect it can have thats pretty good for a lvl 1 spell. Also prevents the ones effected by it from using invisibility.
Entangle also a lvl 1 spell can restrain people who fail the save vs it. Strength save I think? Gives them disadvantage on their attacks and you advantage on attacks against them. And the last effect gives the people restrained disadvantage against dex saves. People keep passing the save against your fireballs? Restrain them first!
Once this effect is gone (or they pass their save) the area still counts as difficult terrain. Which can be annoying for melee enemies to deal with.
Detect magic is amazing. But sadly not in the game.
Fog cloud can make people not see you. On the flipaide you cant see them. But can be usefull *oh crap* button in some scenarios.
Jump and longstrider can be usefull as you have to traverse alot in bg3.
Thunderwave can toss enemies off an elevated postion in some situations.
Lvl 2 stand outs:
Pass without trace. Aoe +10 bonus to stealth checks. When the party wants to stealth with heavy armor on.
Hold person. Clerics got it to. But stunning humanoids can be clutch.
Heat metal. Dont recall exactly how implemented in bg3 but the damage it does isent the main attraction of the spell. You can disarm enemies with it or if they pass the save disadvantage on attacks if they keep holding the item. Makes enemies alot less dangerous.
Spike growth. Like entangle creates diffixult terrain. But this alsp damages them if they move through it. If you take abilities that move enemies backwars (like repelling blast from warlocks for example) you can keep getting mileage out of this spell.
Lvl 3 spells.
Call lightning is nice because you can call down lightning in an aoe for 10 turns. First as a casting of the spell and then later as bonus actions. So you can cast this spell. And shapechange in the same turn (as a moon druid anyway) and then tear things apart as an animal while calling down lightning. Pretty sweet! Can only be used outside though for obvious reasons. (Is that last part in bg3? No druids in my party...)
Most of the other lvl 3 spells id consider good and worth taking arent in bg3 sadly (like dispel magic. Curse you Larian!)
Lvl 4 spells:
Ice storm is a nice decent aoe. Ice isent resisted often and it creates difficult terrain. And its implementation in bg3 is insane because if you slip you just straight up lose your turn. Its worse even. You lose your turn and are now prone giving all attacks against you advantage until your next turn.
Conjure woodland beeing. Its a summon. And in bg 3 it can...summon its own summon?
Polymorph is an insane spell. In the tabletop. Which makes what Larian did to it even more insane. You can only turn people into sheep with it. So no turning yourself into tyranosaurus rexes </3 But thats still an enemy that isent shanking you for the time beeing I guess.
Stoneskin is alright in bg3. Material components arent a thing so the cost of the spell isent an issue. You still run into enough enemies wielding mundane weapons that it becomes a nice buff. Arguably more usefull in bg3 then on the tabletop.
Wall of fire. Aside from the INSANE surfacearea of the wall of fire, its a wall of fire
Hope this helps