Irregular markers (Threat 1, 2 or 3) are all fine. Actually, Threat 1 are the best, because the NPCs are easier to kill, but whatever you find, use that. My search method was populated systems and only the moons around the gas giants. But ideally you'd want a mission POI, so that you only have to re-log (rather than restart).
I am working on what I think might be a better source. The POI is called 'Minor Wreckage' (no threat) and you can get it from Horizons missions called 'Liberate Bootleg Liqour', 'Surface recovery of something during Civil War', 'Surface retrieval of something during expansion' or 'Wartime surface recovery of something'.
I don't know what causes the Liberate missions to appear, but obviously the Surface recovery missions are spawned by factions in the state Civil War, Expansion or War.
The advantages of this method are:
- Because it's a mission POI, you only have to re-log to respawn the data, rather than restart.
- Horizons missions of this type run for 3 to 6 days. This means you can farm the POI for much longer than the similar Odyssey mission POI (Irregular Markers Threat n).
- The POI has two crashed skimmer data ports. I believe this means you get twice as much chance of SDPs per re-log.
- There are never any NPCs to deal with.
The downsides of this method are:
- These missions can take you to different types of POI: Irregular markers, Artificial Structure, Minor Wreckage. You have to visit each planet to find out what POI you will get. If you get a Minor Wreckage, you have to fly down to find out if it is the right type of Minor Wreckage. The type that you want has a wrecked SRV and two crashed skimmers. There are two Minor Wreckage POI types that give you this, but there are probably half a dozen more that don't. If I had to guess, I would say that with average luck you need to visit about 6 POIs to find the right type.
- Sometimes, the data port on one of the skimmers is below the planet surface, and cannot be scanned. So you may find the right POI, but only be able to use one data port.
If you decide to try this approach, when you arrive in a system, use the 'scan the Nav Beacon' method to determine which planet the POI is on. And please let me know how you get on!
This is an example of one of the Minor Wreckage POI types, where the skimmers are either side of the SRV:
View attachment 291999
There is another layout where the skimmers and srv form a triangle, which is slightly quicker to farm.
View attachment 292000
<edit> I should add that skimmers are a great source of other data: Settlement defense plans, Manufacturing Instructions, Maintenance logs, Ballistics data, Surveillance logs, Operational manual and Patrol routes. Particularly useful for Manufacturing Instructions, since you might end up needing those in quite large numbers.