Choice comments based on a stringent methodology include:
Central African Republic: Do not attempt to disprove the four-colour theorem on your flag! (This goes for you, too, Mauritius).
Dominica: Do not put a picture of a parrot on your flag! (This goes for you too, Guatamala!)
Falkland Islands: Worst UK colonial flag. Has a sheep on it. Actually, if you look closely, you can see that the sheep is riding on top of an island, which is riding on top of a ship. Also the stupid slogan is in English, and is a platitude.
Marshall Islands: Looks like an airline logo.
Mozambique: Automatic weapons on a flag are especially bad. Appears to have been designed by a committee all of whom had stupid ideas for pictures of extra things to put on the flag.