Everything here is a complete diet, so it doesn't need anything added. But dry food does need one thing thinking about, and it's the reason this page starts with a caveat rather than a sales pitch.
Cats and water
Cats descend from desert animals and have a weak thirst drive. In the wild they get most of their water from what they catch, which is roughly three-quarters moisture. Kibble is about a tenth.
Plenty of cats manage perfectly well on dry alone and live long, healthy lives. But some don't drink enough to make up the difference, and that shows up later as urinary or kidney trouble. If your cat has had either, or is getting on in years, feeding some wet cat food alongside the dry is worth doing. A water fountain helps too, since a lot of cats prefer moving water to a still bowl.
How to narrow this down
Lifestage first. Kittens need kitten food, and it isn't marketing: growing cats need more of nearly everything. Older cats often need less phosphorus and more support elsewhere.
Then think about how your cat lives. Most cats in this country are neutered and spend most of their time indoors, and both of those lower how many calories they need. Indoor and sterilised recipes exist for exactly that, and they're the mainstream choice rather than a niche one.
Grain free and hypoallergenic come last. They matter enormously to a few cats and not at all to most.
Don't leave the bowl full
Cats graze, which makes it tempting to keep the bowl topped up. It's also the most common reason indoor cats get heavy. Measure the day's ration out in the morning and let that be it. If you feed wet as well, take the equivalent off the kibble rather than adding one to the other.
What dry food does and doesn't do for teeth
Ordinary kibble does very little for a cat's teeth. The dental recipes on this page are a different thing: they use a larger, differently textured kibble designed to scrape rather than shatter. Those can genuinely help. Standard dry food just because it's crunchy will not.
Changing a cat's food
Cats are far fussier about this than dogs and can refuse a new food outright. Mix it in gradually over a week or two rather than swapping in one go.
One thing worth knowing: never try to wait a cat out. A cat that stops eating for more than a day or two can develop serious liver problems, and it happens quickly. If your cat refuses food and won't be tempted back, ring your vet rather than holding firm.
Prescription diets
A good number of products here are veterinary diets for diagnosed conditions. If your vet named one, buy that. They aren't a general upgrade, and some are unsuitable for a healthy cat.