As suggested by Senor Wignall:

Thinking about it a far more sensible way to do it would be to just take the cool parts of the cool flags and construct an uber-flag .

My 5 minute lunchtime effort is: Eagle eating Snake + Two Headed Eagle + Dragon + Maasai Spears & Shield + Two Snowlions + Man behing beheaded:

I think if the rules could be bent to allow non-national flags there could be some awesome mashups as provincial flags and royal standards get all the cool stuff. That said if we were to allow them there’s only one flag we’d need:

It’s a bear holding a freakin waraxe. Thank you Yaroslavl Oblast.

A composite of all world flags weighted by population (and tweaked slightly to make it a little less red)

I wonder if it’s the sort of thing where your your flag just jumps out at you – I can clearly see the Union Flag but can’t see other more populous nations, though it won’t help that nine of them are tricolours.

It’s almost as good as the Earthican Flag.

From a series of adverts for the Sydney International Food Festival:

(via I Believe in Advertising)