Check out this list of foods to be avoided at all costs. It lists the usual suspects (artificial colors, artificial sweeteners, corn, wheat), but it also provides nice explanations about why you shouldn’t eat them.
To me, one of the most important things on the list is artificial sweeteners. I talk to “health-conscious” people every day who tell me they drink diet soda. I never understand how they could possibly think it was less-bad for them than full-sugar soda. I would rather not have any soda, or mindfully drink small quantities of regular soda any day. Diet soda tastes terrible!