I've always enjoyed creating things, and making classes and spells is no different. That said, I've been working on entirely breaking the spell system; obviously, the Judge of the game I'm currently involved in would never allow these spells; for that matter, neither would I!
Take, for instance, this transmogrification spell:
Transform to the form of a living creature. Form limited to particular type of creature. HD limited to caster level and 2x target level. Mental characteristics replaced by new form’s. 1 living corporeal creature. 30'. 1 turn. No saving throw permitted. Arcane.
It bends the spirit of the rules a little, but you end up with a level one, no-save, turn-a-dragon-into-a-newt spell. You can be the guy that killed a dragon by stepping on it.
Wall spells are fun to break, too; this one is a level two spell:
Attacks creatures within 5' as 8 HD monster. Not affected by physical & magical effects. Wall must be bonded to surrounding material. Wall must always be a flat, vertical plane. Wall must always be in contact with ground. Range 0'. Indefinite (until dispelled/destroyed). 100 square feet. 1” thick. May not be evoked where objects/creatures are. No saving throw permitted. Arcane.
An invisible wall that attacks anything that comes near with the ferocity of a huge monster. And did I mention that it's level two? But my absolute favorites are illusions. Breaking things aside, they're just so fun to use (and abuse) in the game. Try this for size:
Create illusion, multi-sensory. Target becomes completely inaudible. Target becomes complete invisible. Illusion limited to 1 obviously fake, harmless category. Illusion dynamic (animated/moving). 1-100 creatures within 120' radius. Self. Concentration. No saving throw permitted (e.g. cannot disbelieve). Arcane.
That is: the caster casts the spell on himself; up to 100 creatures within 120' of him suddenly become invisible and inaudible. However, in their place stands a cartoonish blob; it performs the same actions and speaks the same words as they do, but completely masks who they actually are. This spell not only removes invisibility on multiple targets, but it's also confusing enough to cause targets (say, a pack of wolves) to attack each other!
Finally, to completely and utterly break wall spells for good and always, we have the mime's favorite tool: the invisible, indestructable, and magic-proof shield.
Impermeable to creatures. Impermeable to magic. Not affected by physical & magical effects. Wall may be readily shaped by reducing area. 0'. 2 turns. 100 square feet. 1”. May not be evoked where objects/creatures are. No saving throw permitted. Arcane.
The mime simply creates a wall, 10' x 10' x 1" thick, then shapes it into a portable shield, complete with handle. The shield is invisible, but completely impermeable to creatures, magic (except Dispel and Disintegrate), and weapons. Oh, and it's a level one spell. Shields for everyone!
What are some truly broken spells you've created?