If you have a true static default (IE it's not based on something from another record), the database configuration default value is the proper approach. For example, if you wanted the description to be "Change This"

If you need to do it programmatically for some reason (getting something from the person record for example that isn't supported as a substitution variable), then Travis is spot on that the .NEW script is the most efficient way to do it. This will fire only once per record and will be immediate so users will see it as soon as they click new row. Object save with only the Add option enabled is also efficient but is after the users attempt to save. If you require the field for example, this won't work because Maximo will prevent the save prior to your default. Object init is the absolute worst option because it fires every time the MBO is initialized (on the list tab, on a REST API request, etc.).