Claidemore,
accident/loss rate?
Irrelevant - all aircraft have an accident and loss rate. A fighter's success is not measured in terms of its accident rate but in terms of it's primary job - shooting down the adversary.
True, but some had worse loss rates than others and an aircraft with severe design flaws that is more dangerous to its own pilots than the enemy is not a successful design. I'd throw in serviceability as well since a mediocre fighter in the air beats a superb fighter sitting in the hangar being repaired.
did it meet design specs?
If it didn't meet design specs, it wouldn't have been selected for service. Again, I see this as an irrelevance for this particular thread.
I think this was meant more as "did the aircraft do what the designers/air force intended/expected it to do?"
was it easy and inexpensive to produce?
That's a procurement decision not a measure of success.
What's the point of an aircraft with excellent capabilities if you can't produce it quickly enough to replace losses or in large enough numbers to meet operational needs?