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.
If you're looking for something more offensive, you could go with "What imbeciles" or "What a bunch of imbeciles".
You might consider referring to the specific type of idiocy:
"What a bunch of lemmings" (referring to a long-disproved misconception): a group of people doing something self-destructive, apparently of their own free will.
"What a bunch of sheep": a group of people accepting exactly what they are told by someone else, without thought.
(I'm not sure why the insulting version of these phrases, and Mark's suggestions of "a bunch of idiots", uses "bunch", rather than some other collective noun, such as "a flock of sheep" or "a herd of sheep" or "a colony of lemmings").
Indirectly related, but in the Scots dialect we have the word 'yous' (or 'youse'), which is 'you' plural - "Youse are idiots", "Youse idiots" etc. Handy for all occasions - "youse serious?" "Aye" "Youse are
protected by RegDwigнt♦ Aug 19 '12 at 11:52
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