Some of the healthiest treats are the 100% freeze-dried meat ones. Look for something like
PureBites,
Halo Liv-a-Littles,
Real Food Toppers or
Benny Bully's.
I recently bought some
Evo Wild Cravings cat treats (they're grain-free) and every cat I've given them to (mine, the ferals, my neighbour's cats) loved them. I used them recently to diffuse a tense situation between Aztec and a neighbour kitty. At least they're healthier than a lot of the other choices out there, Greenies and Temptations included.