ISO 27001 does not prescribe which performance indicators should be adopted by organizations, so there is no such thing as best KPIs, and organizations must define them according to their own needs and objectives. Some common issues organizations should take into account when defining KPIs are:
Business relevant: indicator aligned to clear business objectives or legal requirements
Process integrated: a KPI should add the least amount of work possible into business processes.
Assertive: the indicator should be capable of pinpointing relevant issues that need attention.
As general examples we have:
Percent of business initiatives supported by the ISMS
Number of security-related service downtimes
Percent of controls assessment performed
Number of improvement initiatives
These articles will provide you a further explanation about performance indicators and security objectives: