What is the key to effective talent management? One on one communication with the employee or engagement activities? Which has a better impact?

If you have developed the system, and only remains the announcement, so I suggest mixing of activities and meetings.
If not, and it is a change, so change management activities first, then buildibg the system in parallel with informing the people.

Talent management comprises of a gamut of activities such as Recruitment, Retention, Capability Development, Reward and Recognition thus the Talent management strategy needs to be linked with the current business strategy to ensure it is effective.
In my opinion ‘engagement activities’ are only successful if they have involvement of employees thus1:1 is definitely one such platform which should be leveraged on as it has a personalized approach.