"What an idiot!" in plural form
There is a phrase "What an idiot!". How should it be used for a group of people? Looks like "What idiots" doesn't have same emotional meaning.
"What a bunch of idiots" works the same. Or, you can be even more offensive and say something like "what a pile of idiots," though this may not be common in some areas.
Simply saying Idiots would suffice. It's short, immediate, emotionally charged and relatively offensive.