Science Diet isn't the best food available, but when it comes recommended by your vet and you don't know otherwise, it's easy to see why so many people highly regard it as high quality.
I have been told by several different veterinarians that their training in animal nutrition while in school is very brief and often sponsored by Science Diet, hence when these doctors go into practice it's natural for them to recommend the food they are most familiar with that offers a prescription diet.
The most important thing I tell people, many of whom are on a budget is to learn what is pet food and what certain ingredients actually mean. Most expensive does not always mean best quality and nutrition unfortunately.
Earl at DoberDogs has an excellent site devoted to food comparisons. It is primarily aimed at dog foods, but much of the information related to ingredients and understanding the food label works for all pet foods. I recommend visiting and exploring his site if you have the time. Here's the link: http://www.doberdogs.com/foodcht1.html
Everyone's opinion will vary on what the "best" food is to feed your cat. This is an intensely personal choice and you should base it on your own feelings on what is best for your cat, and how well they thrive on a certain food.
In my opinion, the first 3 ingredients are the most important and make up the majority of the product. I prefer they be as natural and pure, free of by-products, meal, fillers, etc...as possible. I personally have a varied group of favorites: the Solid Gold's line of canned cat food, all of the Wellness canned cat foods, Wysong, Innova EVO line - not the Healthwise line though, PetGuard Organic Mature Formula, Azmira Holistic canned cat foods, Eagle Pack Duck & Oatmeal canned cat food (saved one of our rescues from food allergy death), Halo Spot's Stew, Chicken Soup for the Cat Lover's Soul, Honest Kitchen Prowl, Nature's Variety Freeze Dried Diets and Primal Pet Foods Raw Cat Food, though anyone considering introducing a raw or homemade diet really needs to do their research and determine if it's best for their cat's current health conditions.
Take your time and do lots of reading and obtain samples. Many of the food companies above have websites where you can request samples online or email a request and they happily will send you one. You can also find samples of many holistic brands at human health food stores.