You haven’t mentioned what tests your vet has performed. A cat that age should certainly have kidney and liver function checked (keep in mind, kidney values won’t show any increases until at least 80% of kidney function has been lost). You should also check her thyroid levels – older cats often become hyperthyroid and lose weight in spite of a good appetite. A thorough work up (radiographs and bloodwork) is your best bet. An appropriate diet can be recommended based on the results of the tests. At the very least, she should be on a senior or reduced protein diet. Any good quality diet is fine for the youngster (Science Diet, Eukanuba, Iams, etc). Be careful about bouncing around between brands – you can develop a picky eater that way. Pick a good diet and stick with it.