Stoneskin - the original version. Its like mirror image in that you're completely protected against a certain number of physical attacks. Its better than mirror image because you can cast it on someone else (like a front line fighter) and there's no chance of hitting the 'real' you as there is with images. Great duration too. We abused this regularly in PnP games. We often had guys running around with Stoneskin up and a contingency Stoneskin that would activate when the current Stoneskin was used up. We can add Contingency to the list come to think of it. I recall fights which went like 'you take those 3 Fire Giants, all take these 3'. We had guys who could take a giant down in maybe 2 rounds, so they wouldn't even get through the layers of protection we had.
Other noteworthy spells: Protection From Fire (Druid), Protection From Lightning (Druid), Polymorph Other, Fireball, DBF, Magic Missile, Phantasmal Killer, Prismatic Spray. In one game I got high enough level to enjoy an awesome 9th level spell called Shape Change. Nothing like changing into any creature you can think of.