Shadow Dragons, definitely. Nothing says, "We're screwed!" Like a party of adventurers taking multiple negative levels, even if they make the save, losing their highest level spells, and then losing effective caster levels so their enfeebled spells won't even be able to beat SR.
I think our group would have died facing a shadow dragon if I hadn't used Lesser Planar Ally to call an avoral to help deal with the dragon and her minions. After it breathed on 3/4s of the group, including all spell casters, we couldn't do much of anything. And that was the first time the wizard and sorceress actually held back on some spells to save for the dragon.