Rawz single protein pates. Simple recipes with no nonsense ingredients. These are the gold standard for food trials. Wet Archives | RAWZ
Simple recipe = meat, moisture, organs, and supplements (ideally in that order)
No nonsense = no grains or starches, no fruits or veggies (pumpkin excepted for its digestive benefits), and minimal gums and thickeners
Of the gums, guar gum seems to be the most benign (least harmful.) Carrageenan and agar-agar are the most irritating and should be avoided. Xanthan gum is one to watch for. It makes smooth foods smooth. It can also make poops smooth to those sensitive to it. Those lick tube treats like Churu and mousse foods like Tiki Cat's Velvet Mousse have xanthan gum in them. With my Krista, it was mousse in, mousse out.
One other ingredient to pay attention to for cats with sensitive tummies is anything with glucosamine, chondroitin, or green-lipped mussels. For some reason, these tend to be irritating to many cats.
I'm giving you all of this so that you can read labels if Rawz isn't available to you or you want to try something cheaper.
On the cheaper end, assuming your kitty doesn't have any protein sensitivities (IBD), you could also try Fancy Feast Classic pates in the non-fish flavors (chicken, turkey, or beef) or Sheba Perfect Portions (also in non-fish flavors.) Fish can be irritating, inflammatory, and addictive. The only thing I see wrong with those two options is the inclusion of tapioca starch in Sheba and the mixing of proteins in both. Assuming this isn't IBD and there aren't any proteins your kitty is sensitive to, you can do a lot worse with a lot more nonsense ingredients.
A nice mid-range food (something higher quality than Fancy Feast but not as boutique as Rawz) would be Tiki Cat--also in the non-fish flavors. Either a simple chicken and pumpkin from the Aloha Friends line or something from their After Dark line.
On fish: it can be irritating and inflammatory. It is also very popular and can be addictive--your cat may hold out for fish flavors. For this reason, I recommend saving fish-flavored foods for disguising medicine or restarting a sick cat's eating. If you feed fish on the regular, you lose this "nuclear option." Fish broth like what's in Fancy Feast Classic pates is okay. Fish oil is also okay. It's foods that feature fish protein aka some kind of fish is in the flavor name (chicken and tuna, chicken and salmon, salmon, shrimp, etc), those should be avoided except in exceptional times (disguising medicine and getting a sick cat to eat again.)
I had excellent luck with this for one of my cats a few years back before she passed. She could literally eat nothing else. One of my current cats has food allergies and I had to go with RC Hydrolyzed.