Having played a druid in the Living Greyhawk campaign for over two years I can tell you that there are spells at every level which are a great asset to any party/caster.
1st Level:
Hide from Animals-It not only makes you invisible to the guard dogs, but they cannot even detect you through any sense other than touch. If you attack someone the effect is broken, but this allows you to get your foot in the door.
Faerie Fire-See invisibility for druids. If you are like most druids and have buffed your listen checks you can negate invisibility for the entire party at up to long range.
Pass without a Trace-Yeah it doesn't help you any, but it is invaluable if the entire party is trying to get somewhere without being followed (like away from the BBEG)
Produce Flame-A good ranged attack spell for druids who normally don't have much to offer from range.
Entangle-Huge area of effect, doesn't kill or harm those that get trapped, and it can drastically change a battle. What's not to love. Oh right...the fact that it will only work for you when you're outside.
2nd Level:
BARKSKIN-This is the #1 2nd level druid spell. Until I could cover the entire party with them I didn't memorize anything else.
Resist Energy-#2 2nd level druid spell. Mem one of these to give yourself resistance 20 to an energy type. This can drastically alter a combat with an elemental or energy using creature.
3rd Level: (I would note that this is where summons get fun so these are all "filler" spells to hold space for Summon Nature's Ally III)
Call Lightning-No reason not to have it is what I say. If you're anywhere but underwater you can cast this before going into a battle and with the minutes/level duration whenever you find yourself without something immediate to do in the combat you can "call down the thunder" as it were. Also, since you've already cast it and the spell is active you could argue to your GM that they wouldn't know that you called the bolt! If they know much about spells and you're obviously a druid then it's a no-go, but for those BDFs they won't redirect just to attack you most likely.
Quench-ever been in a situation where there was a massive dock/warehouse/etc. fire and you had to work your tail off to put it out? Well, Quench is the answer. Look at the area of effect. Also, if you're fighting fire elemental creatures then watch out because it's d6/level w/ no save.
Plant Growth-Battlefield control at its finest. You can basically make your own hedgemaze and route enemies exactly where you want them... that is of course unless you simply want to trap them in it to slow them down.
4th Level:
Covered by the others. I like Air Walk, Flamestrike, and Freedom of Movement best myself.
Good luck and good hunting