Breach.
You can either focus on farming breach splinters (up to 200 per map in my experience), which I personally find boring, or you can focus on catalysts or a combination of both.
I don't know what's the optimal strat for farming catalysts but I focus on buffing rares (+rare monsters, effectiveness, extra modifiers, delirium, etc) and make about 5-7d per map on average. I just did a map where I dropped 14 neural catalysts (about 18d); though this is rare, it's not uncommon to drop 2-3 neural/sibilant/reaver catalysts per breach opened.
Also, running the splinters is extra profit, especially if you're willing to reforge the low value catalysts.
Edit: Do note that catalysts are steadily dropping in price. I don't think this is because people are switching to farming breach, as I have some pretty insane breach tablets for sale (in some cases being the only seller for that specific combo) and they're going pretty slowly, but at the end of the day this farm is still losing value.