Is DB2 more reliable and stable compared to PostgreSQL or MySQL?
All of the platforms you have mentioned can run close to zero data loss configurations. All of them could be deployed in a configuration that will fail.

Platform choice is one part of the puzzle. It will be your implementation of the platform that determines whether or not your requirements are met.


MySQL Cluster
Oracle RAC
PostgreSQL High Availability
SQL Server High Availability


In reality, high availability and disaster recovery are a trade off between uptime, Recovery Point Objective, Recovery Time Objective and cost. 

Zero data loss and zero downtime is expensive, complicated to setup and complicated to maintain. There is no free lunch.