I love this post by Angela at Adventures in Self-Reliance. She got samples of most of the major powdered milk brands, conducted a taste test, and posted her results. She has some great information.
As you can see from her ratings, personal preference varied widely (with a few brands more broadly liked or disliked). As her tasters sampled one of the brands of milk, they were surprised to find out that they didn't like it. Angela writes, "I had some folks about cry when they found out how bad it tasted because that was the only kind they had stored."
Once again, this shows the importance of tasting the powdered milk that you store!