.22LR overall has the best "bang for buck" or "fun value" because the result is very quiet (on an absolute scale), cheap to shoot, fun for plinking, useful for pests, etc.
Centerfire pistol suppressors are somewhat novelties-- cool novelties, but still novelties.
A suppressor on a .223/5.56 AR-15 shooting full power ammunition is still pretty loud due to pressure coming through the action. There are also heat and fouling disadvantages.
Suppressors on precision or long-range rifles are typically an advantage for shootability and accuracy.
For those reasons, I'd give the following rating for usefulness or my own priority:
1. .22lr suppressor
2. precision/long range suppressor
3. centerfire pistol / AR-15, it's a wash
Of 9mm and .45, the 9mm with subsonic ammunition will be quieter.