I recommend using a nutrition bar for a healthy snack rather than as a meal replacement, so you’ll want to stick to 200 calories maximum. Aim for 12-15 grams of protein and about 6 grams or less of fat and no trans fat. And choose one with only 1 gram or 2 of saturated fat if you have high cholesterol levels.
Remember too that the ingredients are listed in order by weight so stay away from any bars that have any form of sugar, corn syrups etc. listed first. The first few ingredients should be various forms of complex carbohydrates or protein such as rolled oats, oat flour, whey protein, soy protein and brown rice are all examples of acceptable ingredients.
Here’s some nutrition bars that are available in most grocery stores that you might want to try out (Source: FitnessRx):
Atkins Advantage Morning Oatmeal Raisin: 140 Calories
Atkins Simply Good Natural Honey Oat: 150 Calories
Pria Complete French Vanilla Crisp: 170 Calories
Luna Toasted Nuts and Cranberry: 180 Calories
EAS Myoplex Lite Chocolate Chip Crisp: 190 Calories