My favorite spells tend to be utility spells, those with multiple functions and can be fun for the clever. These spells are great in of themselves as well as being templates for greater and lesser versions. There is only 5 and 3 of them are in the SRD (one is a first and second edition variant that didn't make it into third). From lowest level to highest:

Unseen Servant- I think this spell should have been on the 1e illusionist's spell list. Beyond being an extra set of hands, a trap springer, a maid (it does mention mending and cleaning) and a thief's best friend, this spell has one huge advantage that allows its use in combat. The servant can only be slain by magic. So, hand it a mundane bomb and it will not be affected by the explosion. Of course the range is not great, but that is what Far Range or a higher level version is for. This spell is better than any form of telekinesis as it works on its own, requiring no concentration. An 6th level version should be able to lift at least a ton and so could be used to remove the ever present rubble in 1e dungeons as well as dig allies out of cave ins. Another variant allow the servant to use a weapon. Multiple castings allow for indestructable (to mundane forces) bodyguards. A 2nd level version could help in the lab or with basic crafting. The basic spell could be used as a very limited version of Animate Object and higher level versions could use frames that seem to be golems, undead or animated statues. Unseen Servant really is a handy, astounding tool. I want one.

Special Effects- this is a 4th level spell from 2e. I found it in The Wizard's Spell Compendium and have no idea of its original source (anyone know?). It is an Alteration that permanently changes one sensory quality of an object, creature or location. So simple, so powerful. One use I found for it is the orc rock. This sling stone glows purple and green, buzzes like electricity and feels very cold (so 3 castings). Superstitious creatures and people freak out when it lands near them. Another is the haunted chamber or tunnel- low curses in the language of the local tribe coming from the walls and air keep them out and allow the party to rest (or draws them in depending on how well the caster knows the language and the people/monsters). Of all the spells that can make money, this may be the greatest for lower level characters. Want to keep your poodle up the standards of the week? This spell allows you to, even if the standards are really bizarre.

Fabricate- I am not a fan of the basic uses for this spell, but the exotic uses that the DM allows can result in some pretty amazing things. Like a miniature solar system in glass- the raw materials are a sunstone, some bits of metal and sand. With some other spell (one I have never seen), it could even be animated, the planets slowly passing through the glass. Or a lich's goblet that contains a tiny zombie goldfish in the stem, swimming up and down forever. That just needs sand or lead crystal. How about fake dragon eggs made from the ground of the nest and colored and textured with Special Effects? Or creating a body for a golem- this should reduce the cost (if the DM allows). Another use, one that I actually did in one game a long time ago, was taking salt out of saltwater and creating a wall around an ooze. Or the time I accidently caused an avalanche (didn't know the wall was the side of the mountain). The spell looks meh, but in the right hands, it can cause major chaos.

Reincarnate- this is the magic-user version, not the druid. I have mentioned the spell before when I wrote up some new feats to allow 3e arcane casters to use the spell and come up with their own lists of possible creatures. I like this a lot more than Polymorph Other as it is impossible to resist and the results can be quite humorous or deadly.

Polymorph Any Object- As useful as this spell is, I like it as a template for a bazillion weaker spells. Polymorph Swine to Dire Wolf should be a 2nd level spell, especially as the wolf is not controlled by the caster. The 5th level Polymorph Steel to Lead allows the caster to seriously weaken foes as it affects one suit of armor (or other object) per caster level. A 4th level version could allow characters to become lycanthropes but with a time limit- they can't spread the curse and will be back to normal after a few rounds or hours (though that might be too powerful when compare with Tenser's Transformation). Right now the possibilities are overwhelming. One idea I like is using it with Clone. I am not a fan of the existing version of Clone in any edition. But it allows for high level casters to do something extraordinary. Create a clone and then use Polymorph Any Object to turn it into a monster. The caster then kills themself when the clone awakes, avoiding the insanity issue or transfer the soul and the clone takes over the caster's life- as a dragon, titan or other powerful monster. The clone's innate form is the new form. That means if they use Shapechange and then die, they revert to the new form, not the human(oid) that started it all.