Creepy Costumes: Zombie, Corpse Bride etc.
These will require copious amounts of makeup, cutting and designing.
Superheros: Batman, Spiderman etc.
These will have a mask that always has eye holes that are too small or else it doesn't fit properly. The child will end up wearing the costume without it and it will be hard to tell what he even is considering he will probably be wearing a jacket.
Princesses: Elsa, Cinderella etc.
Pray it isn't Elsa they want or you will never find your child amongst the sea of other Elsa's at the school party/parade. You will then take a picture of all the Elsa's and delete the ones that are not your child later. Try to talk her into Snow White or Jasmine which will not be as popular.
Disney Characters: Mickey, Minnie, Toy Story, Monster's Inc. etc.
Everyone loves classic Disney costumes. No qualms about these.
Animal Costumes: Ladybug, Monkey etc.
The hood will either be too small or large, covering the child's eyes so you can't use it. Then no one will know what your child is. The will just look like they are wearing a sleeper or a sweatsuit.
Food Costumes: Pizza, Banana etc.
I do not understand the allure but whatevs!
Something too skimpy/not age appropriate: french maid, adult TV characters etc.
The kid will freeze to death or people will judge you for letting them be Heisenberg from Breaking Bad. Parent of the year!
Something you have no idea what it is: Finn, Beast Boy, Rigby etc.
HUH? These guys are on the shows you don't watch with your kids because they are super annoying, plus isn't TV supposed to babysit your kid while you make dinner and do housework? (Don't judge me, you know you do it too!) Now you need to spend aaaaaalllll night googling these guys to find out who they are and where the hell you are supposed to find this costume!
Some Crappy Annoying Character: Spongebob, Caillou, etc.
HELL to the NO!!!
Something with a Buttload of Accessories: Wand, Sword, Jewelry, etc.
Seconds after you leave down the street you will be asked to carry said accessories and if you have more than one child it will not be pretty. Either you do it or you listen to whining the entire time about how "it's tooooo hard" or "I can't carry it AND my bag"! UGH!
Good luck parents getting your kids to be what you want them to be (for Halloween that is). I despise trick-or-treating so I get costumes on clearance sales after Halloween each year when they are not with me and then they have to be whatever I say! MUUUUUHHAAAAHH!