A big myth is that bananas are high in potassium, magnesium, and calcium. If you eat 1 medium banana you're getting 422 mg of potassium which is only 12% of daily value, 8% for magnesium, and a bit fat zero for calcium.
To put this myth in perspective, there are literally hundreds of foods which have more potassium than bananas. A 200 calorie serving of raw spinach provides 4,915 mg (over 10x the amount). Summer squash is 3,275 mg. Brussel sprouts are 1,805 mg. Sure those are vegetables, but if looking at fruits there are 70 which rank higher than bananas including melons, guava, kiwi, apricots, white grapefruit, nectarines, and dozens of others. So are bananas healthy for you and should you be eating them everyday? There's nothing wrong with that so long as you realize their nutrition facts and how they have a somewhat low amount of antioxidants when compared to many other fruits, such as all varieties of apples and most berries.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010