What Single Property Best Defines Quality in Software?

The most important thing in the good software is maintainability. A good software not only should be maintainable but should also rely on rapid development and iteration without losing reliability. Should also be easy to modify and extend and enabling fast delivery of features and updates. About its design, it should use clear structures, well-documented code, and should rely on automated testing for seamless updates and modifications while ensuring reliability and performance. Ultimately, it will empower teams to deliver value to users faster and more efficiently.

On the other side, bad software lacks all these attributes, leading to slower development, increased technical debt, and a higher probability of bugs.

Every “good” software possesses reliability which “bad” software lacks.

​@DaveFarley 
Reliability stands for various important aspects including:

Consistency: A good software performs as expected without any unreasonable failure, abnormal termination, operational errors, or any performance issues. Users can count on it behaving the same way every time.
Precision: It works as expected and provides the right results in accordance to the requirements and business logic.
Recovery: Reliable software would be able to cope with extreme conditions, user error, and would return to working order without any serious interruptions after crashing or halting.
Performance Stability: Good software whose response is quick and stable irrespective of heavy workloads and complicated work processes.
The Significance of Reliability:
Reliability affects user confidence and satisfaction. No matter how feature-laden the software is, its unreliability can be quite annoying, while a simple but well designed, reliable and stable system can go a long way in establishing and sustaining a good image for the company.

To sum it up, reliability is usually the feature that distinguishes software of good quality from bad. It shows proper design and thorough testing with due diligence to quality throughout the software development life cycle.