Flow — full stop. Salesforce has put Workflow Rules and Process Builder on the retirement path. New automation should be built in Flow Builder.
The history matters in interviews:
- Workflow Rules were the original tool. They could update fields, send emails, create tasks, and send outbound messages — that's it. Simple, fast, but extremely limited.
- Process Builder came later. It could do more (call Apex, create records, branching) but had performance issues, was hard to debug, and is now end-of-life.
- Flow does everything both of those did plus much more: screens, loops, calls to Apex, HTTP callouts, before-save record-triggered flows for fast field updates, scheduled paths, error paths, decision elements, sub-flows.
So in a brand-new build, you reach for Flow. For existing orgs the conversation shifts: there's a Migrate to Flow tool that converts workflow rules and process builder processes to flows automatically, with manual cleanup needed afterwards. Salesforce has announced a hard retirement timeline, and orgs that haven't migrated yet are running on borrowed time.
A nuance: for record-triggered flows, prefer before-save flows to update fields on the same record — they run an order of magnitude faster than after-save flows because they skip the second DML round trip.