I like Purina One, which comes in a few formulas--I think Senior, Indoor, Sensitive, and Weight Control. However, my male Seal Point Siamese vomits when eating any of them. He's 8 years old and has some sort of asthma problem and what I feel is an allergy to grass. He gets shortness of breath and the vet gave me theophylline and prednisone to give him. He vomits a lot. Sometimes it's from not masticating his food. But other times I think he vomits because mucous gets into his stomach which causes him to vomit. I've been told that Siamese have a tendency to vomit.
Now in answer to your questions.....I, too, have tried Purina weight control dry food with hairball treatment and Purina One for sensitive stomachs. The cat, Junior, still vomits. We fed him and his half-sister Mika Fancy Feast since they were kittens, so it's doubtful they would stay on Science Diet for more than one meal because it's so bland in comparison. I should try iams with them and see what happens. I also have a 4 year old black and white stray with fluffy hair. She, Sydney, about whom I wrote a Hub, hardly every vomits and I give her Purina morsels for hairball treatment. If she vomits, it's usually to throw up a hairball. Junior licks himself constantly, so I have to make sure his hairball problem is controlled too. I really have to make a point to take pictures of all 3 of them to make my Hub pages more interesting.
You mentioned all your cats have very dry skin with white flakes. You should call your vet's office. Why would all your cat's have this dry skin condition? Is it their diet? Is it the climate you live in? I would suggest that you add a little (very small amount) Wesson oil to their food or have them lick it or butter from your hands. I do this when my female Siamese eats grass and seems to be constipated. You should check with your vet first for professional advice, as s/he may have some other suggestions for you to address the dry skin problem.