RSE, JT400, CoreIQ, or a file upload for this one process?

We only need the programs for one process. Which import should we use?

Use the source that already holds them. RSE or JZOS for live z/OS members, CICS CMCI when you are discovering programs, JT400 for IBM i libraries, and CoreIQ or a Hub planner export when that catalog already has the estate. A file upload or a .atq export works when you cannot reach the host. Import the boundary you will modernize, not the whole machine.