I'm going to assume that you aren't looking for vitamins to absorb anything, but that you are looking for something that can absorbed by your body.
There are 2 parts to absorption.
One is, is the vitamin delivered in a fashion that allows it to survive the digestion and elimination process? That is, does it survive exposure to stomach acids but does it break down enough to pass through the walls of the small intestine and into the blood stream. That has to do with the form it's in. So pills have a very low "survival" rate - you can google a zillion studies on the concentration of undigested or partially digested pills (prescription or supplements) in septic tanks, sewer system filters, and public water supplies.
The other is, is the product usable by the human cell once it arrives there? Because no nutritional element works by itself. So, for example, those "vitamin drinks" that have 1, 2 or 3 vitamins are a complete marketing scam. They accomplish nothing (short of hydration that you could get with tap water, but that value is offset by all the sugar and dye). Combination is the theory behind combined multivitamins with minerals, but most of those don't contain nearly enough ingredients. So they remain questionable in value.
There was a theory some 30 years ago that people should take only those vitamins or minerals they are "low" in. So if a blood test showed low iron, the doctor wrote a prescription for iron. Linus Pauling hawked Vitamin C for colds (debunked many years ago, but people still repeat it like it's science). People in northern climates were told to take Vitamin D. This micronutrient approach has been a virtual failure because we are far more unhealthy as a country than we were when it started. There are still some old-school doctors and nutritionists who still recommend single nutrient approaches, but it's very much fallen out of favor. But vitamin aisles and separate stores have exploded and proven to be extremely lucrative for the manufacturers and distributors, so no one's going to cut back on it despite the fact that no research shows it to be the most effective method.
Yes, as mentioned below, nature should provide us with what we need in our food. We're told to "eat well" and we will be well. In theory, natural foods do indeed provide all we need - with ALL of the necessary nutrients and not just a few vitamins and minerals someone throws into a pill - but that's not the reality any longer with farming techniques in the US as well as in other countries. The foods are nutrient-deficient. The AMA came out over a dozen years ago recommending that absolutely every American supplement.
Supplements come in 2 categories: there are vitamin products that are considered drugs and are approved by the FDA (as are prescription meds, typical OTC things like pain relievers and cold meds). They will all have warning labels on them. So Flintstones vitamins say to "keep away from children" and adult products will have strict dosages on them.
The 2nd category is food-based products supplements. These are not eligible for FDA approval because they are food. (There is no "FDA Approval" on your spinach and cheese either; there's also no FDA approval on products with a 40-ingredient nutritional panel such as processed foods, some breads, etc.). So the absence of FDA approval doesn't mean there's anything wrong with the product, just that it's not considered a drug with a potential for overdose. Supplements in this food-based, non-drug category are required to be labeled that they are "not intended to treat, diagnose, cure or prevent" a disease. That doesn't mean they don't have value. They are regulated by the FDA but not "approved" as a drug is.
Does that help?
There are plenty of ways to evaluate a product and a company, depending on what your priorities are. There are a number of excellent supplement companies. Things you might want to consider are whether they use GMO ingredients or not (some people care, some don't), whether there is a patent (which proves uniqueness and a guarantee that what's on the label is in the product - you may have seen the recent reports on things sold at places like GNC and Whole Foods that don't even contain what the label claims), whether it's made in the US or just distributed by a US company but made elsewhere with lower standards, whether it's designed for children or adults, how the Chief Scientific Officer is rated, what the reliance is on clinical studies, and other factors.
Without knowing your priorities or even whether this is for you or your children, I can't be more specific. But that's an overview for you to consider.
@Christy Lee - thanks for the ringing endorsement! But I have to correct your statement that I sell vitamins. I'm a long-term educator (in health care, the schools, and nutritional epigenetics) and I've been very consistent in trying to educate people about NOT taking a small mix of vitamins or vitamins/minerals, for the reasons listed above! They are not absorbed at the cellular level in these typical formulations available at most stores.