Optimum nutrient has a huge fan base. Lot of people like their whey powder too. Personally I go for the cheaper stuff. Whey and vitamins. For vitamins just stick with a brand name and you'll be fine. Se below article which makes sense to me.
There isn't really a "best"...
A vitamin is a vitamin. If the vitamins you're buying are pure and the dosage and ingredients listed on the bottle are accurate, it's as good as you're going to get. While there could be some discussion about which types of vitamins or minerals are best (for instance, whether you want to choose calcium citrate or carbonate...or calcium by itself or calcium in combination with magnesium or vitamin D), in a perfect world, one brand is as good as another. 500 mg. of calcium citrate is as good with one label on the bottle as it is with another.
The thing to keep in mind, though, is that vitamins and other nutritional supplements aren't regulated by the FDA. So what the bottle says is in the bottle could be different from what's actually in the bottle. I'd venture a guess that many supplement suppliers are honest. I work for a company that manufactures lots of different consumer products and we have a line of dietary supplements. We do independent testing of every single batch to make sure they're up to standard. If word ever got out that a dietary supplement maker was fudging the details on their labels - or if their QA was so bad that there was variation in the strength or concentration of the ingredients - their reputation would take a huge hit and no one would ever by their products; there's at least some incentive to do things right!
In my experience, if you stick with the bigger, nationwide brands - the ones that have the most to lose by cheating - you'll be fine.