Pretty much every splatbook for 3.0 has broken spells. So many are broken or unbalanced that IMC we don't even allow non core books. One that stands out for me though was Ghostform from Tome & Blood. I don't have the book in front of me but it basically made the caster incorporeal, with all the benefits and none of the drawbacks. 50% chance to be immune to even magical effects, totally immune to non-magical attacks/effects but still able to 100% affect the material world with your spells. Also allows you to fly and go through walls as well. Nice utility spell by way broken if you ask me.
Gate is very broken if you allow the caster to summon things up to twice their HD, what was the logic behind that? Minimum caster level is 17 for this spell so that's 34HD. That's a bit over the top. Yet if you are summoning multiple weaker things the total can only be equal to your HD. 3 wimpy things does not equal one super uber thing by a long shot. I can't beleive it was never erratted like Shapechange to only be equal to your HD.
Cone of Cold does kinda stink. Cones in general kinda stink as the caster needs to be at the front of the fight to use it without affecting teammates. How often does that happen? My sorcerer does not like the front lines, does yours?
Mind Blank does seem broken. Lasts all day and gives total immunity to an entire school of magic, to 2 schools of magic depending on how you interpret it. Definitely at least to 1 and 1/2 schools of magic though

I hate to say that though as my sorcerer uses it a lot. Hope my DM isn't reading this....
Geas? No Save, 6th level spell you are now my monkey boy. Give an order like travel around the world and off they go. Done deal. You can defeat any monster if you can get past their SR. At 11th level. Wow.
Miracle. Where do I start. It's like wish, but you don't have to pay 5K xp unless you REALLY go overboard with what you want. Why do arcane casters have to pay 5k xp for any use of it, while clerics get it for free basically?