Tiny Hut.
Why? It's got some nifty properties, useful for virtually any ranged attacking:
1) it's got a 20-ft radius - so a 40-ft diameter, and is thus 7-8 squares across.
2) It doesn't interfere with the vision of those inside
3) It's opaque to those outside (you have total concealment! Yay!)
4) Standard Action
5) 3rd level
Thus, in combat, you can summon it, and take a five-foot step (or even a move action - you can be anywhere in the hut in the same turn you make it) to cover most, if not all, the party with total concealment. An attacker (who isn't using area attacks or some such, anyway) not only has to guess at which square you are in, but has a 50% miss chance even if the attacker picks the right one (and there should be about 50 of them to choose from, in a fully open area (pi*r^2; r = 4 5-ft squares))! Even better, at that level, it's the only spell that covers most the party in one round AND doesn't interfere with the party's actions. Sure, your opponents can charge in and see as well as you can - but then they are moving into an area where they don't know the layout of the party, and any non-area abilities are virtually useless from outside.