USAF has highest F-16 serviceability in the world .
All other operators lower.
@Bilal Khan (Quwa) @Gryphon @Tipu7
As far as Availability of certain Defense equipment is concerned, it depends on two vastly different parameters.
Tangible & Intangible.
The Tangible elements of any military hardware are related to nature of that weapon system, availability of supply chain, quality of material, presence of relevant storage & maintenance facilities and availability of budget for affording such measures.
The Intangible elements are related to training of maintenance crew, activeness of threat environment, status of that hardware in military doctrine.
If you compare United States with Pakistan, USA is easily way ahead as far as tangible elements are concerned. However, if we consider intangible elements then this is where American dynamics are vastly different from those of Pakistan's.
For example,
1: USAF has largest number of air crafts of all types around the globe. Their is no need to actively focus on their operational availability as USAF will always has more than sufficient number of air crafts to tackle any threat. PAF however, operates very limited number of advance air crafts (18 Blk52, ~45 EMLUs) and it face threat of much large magnitude. Thus as a compensation factor, PAF actively focus on operational availability of its F16 fleet.
2: USAF threat perceptions are different. They enjoy strategic depth and I can assure you that USAF Fighter fleet deployed at its mainland has pretty much low operational availability as compared to those deployed at overseas bases. Thus due to 'relaxed' conditions as far as USAF deployed assets on US mainland are concerned, they can afford low operational availability while those of forward bases deployed across the Globe close to threat cannot. PAF case is also pretty much same as those of USAF overseas assets.
For PAF, active readiness is something which give it some tactical and strategic advantages against much larger and more capable IAF. For reducing quantity edge of IAF, PAF has inclined to enhance operational readiness of its available assets. For reducing quality edge of IAF, PAF has extensively focused on battle-centricity for reducing fog of war and achieve precision in their objectives.
I guess this is sufficient explanation. You can now easily understand why European nations despite of all their sources have pretty low operational availability for their air crafts while those like Pakistan, Israel etc have kept it as high as possible.