Hi Ladga,
You only need to select Sort newer to older in Account's Timeline to see the latest related activity. This also includes activities of related entities such as Leads, Contacts or Opportunities.

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If you need a field to show the latest date, I would recommend you use several workflows instead of a rollup field.
Please refer to this article for an introduction to workflow: https://crmbook.powerobjects.com/system-administration/processes/workflows/
1) Take Email as an example, here are the detailed steps.
Create a custom Date field for each of the related entities such as Leads, Contacts or Opportunities to indicate the latest activity date of this entity.

Choose the right Scope. select Start when: Record is created.

Add a step: Check Condition. Set the conditions as Regarding (Lead), Lead, and Contains Data.

Then add a setp: Update Record - Regarding(Lead). Update the custom Date field to the date of the Email.

Add a step: Check Condition. Set the conditions as Regarding (Opportunity), Opportunity, and Contains Data.

Then add a setp: Update Record - Regarding(Opportunity). Update the custom Date field to the date of the Email.

Add a step: Check Condition. Set the conditions as Regarding (Contact), Contact, and Contains Data.
Then add a setp: Update Record - Regarding(Contact). Update the custom Date field to the date of the Email.
Set up all account-related entities like this.
Activate this workflow.
2) Create a second workflow, take Lead as an example.
Workflow is triggered when Lead's custom date field is changed. Add a step to update record, update the date field of the related Account to the custom date field of Lead.


Activate this workflow.
Through these two workflows, we can display the latest activity date of the relevant Lead on the Account form.
You also need to create similar workflows for other active entities and entities such as Leads, Contacts or Opportunities.