Value and importance of spells varies wildly by build, but these are the standouts IMO;
Level 1: Ablative Armor, Conjure Bolts, Master's Touch
Level 2: Elemental Weapons, Flame Arrow, Insightful Strikes
Level 3: Insightful Damage, Positive Energy Infusion, Stoneskin
Level 4: Armor of Speed, Greater Elemental Weapons, Remove Poison
Level 5: Remove Curse, Radiant Forcefield
Level 6: Deadly Weapons, Tactical Detonation
Obviously, repair spells become important if you are a construct and/or in a party with others. Attack spells (e.g. Blast Rod, Arcane Tempest, etc) move up in value if you are caster focused with high spellcraft values.
I don't see a lot of other artificers using Positive Energy Infusion, presumably because it nerfs your HPs. However, if you are playing in a group, casting that on the party tank can make a huge difference.