Most spells that gives you continuous and enduring benefits for a single casting (except for those that merely restore the status quo), such as continual flame, explosive runes, animate dead, planar binding, wall of iron and so on. Granted, not all of them are broken, but the ability to, say, single-handedly raise armies with a one-time cost is incredibly unbalanced, especially since the resources are exceptionally easy to replenish.
Greater/teleport and similar spells are extremely broken, especially when coupled with divination spells. Scry 'n die is a broken tactic that's really hard to overcome on a continuous basis. It's far too easy to assassinate people when you can pop in unannounced, kill everything in sight, then pop back out again.
Also, a number of other spell-combos are broken, even if the individual spells are alright.