What is the best cat food brand for cats with sensitive stomachs?

I have a siamese cat who has a lot of digestive problems. He vomits quite often, and the hairball medications does not seem to completely get rid of the problem. Also, at 9 pounds, he could use some extra weight. I was wondering if anyone had any insight on the best cat food to give him, so that it does not irritate his sensitive stomach. Thanks!