The best food is the one your cat(s) do the best on, regardless of what anyone else thinks about it. We've fed foods from Purina to Innova and have found that our cats seem to do the best on mid-grade foods. In our case, cheap foods = poor coats, big&smelly stools, poor teeth, etc. However, most of the super premiums tend to be too rich for my older gal and caused excessive weight gain & loose stools.
So, we settled on Iams Weightloss + Hairball combo for the house cats and Iams Original for the outside cat. They also get a variety of wet foods (Iams, Nutro, Science Diet, Pro Plan)~ approx 1oz in the evening. This works well for us and keeps everyone happy, healthy and in good shape. We also really like Eukanuba, but it's higher in calories.
The main reason for senior food is the less calories. You also want to watch Phosphorus levels for kidneys. Certain vitamins/minerals might be increased, glucosamine may be added for joint health, and kibbles may be smaller&easier to chew. However, if a cat is doing well on their regular formula, IMHO, there is no reason to change them. Talk to your vet about it and access your cat's condition-you ultimately have to decide what to do.
Now, for the real question-what's kitty's favorite treat?