Do you feed wet food or dry food? I have two cats and one has mainly wet food and a bit of dry food and the other prefers mainly dry food and a bit of wet food.
The wet food of choice for mine is the Felix As good as it looks (~£15 for a box of 48 pouches from Tesco). Whiskers was a brand my eldest never really took to. She was rather smelly on it. The cheaper version of felix wet food (~£12 for a box of 48 pouches) isn't enjoyed as much by my girls. In my more financially challenged moments I once bought a box of Tesco's own brand cat food, used most of the box and then read the ingredients, I threw the remaining ones away. You know they actually list "Ash" as an ingredient? (if you look at the analysis of most cat food ash is listed but with tesco's own it is listed as an ingredient) With the wet food I buy Go-Cat indoor cat dry food.
I find that (with my cats at least) the litter tray is less smelly if their diet has a dry food element in it. I also find that although the pouches work out slightly more expensive they are better. Less smelly, less jelly and generally more loved by my cats.
It is trial and error to find the food your cats love but remember that changing the brand you feed them can set off an upset stomach and possibly diarrhea, this usually clears up within a few days though. This could leave you thinking that a food makes the litter tray smell worse when in actual fact it is the change in diet that has done it rather than the new food.