Farm has the largest key economy of the starter sectors and the widest spread between the best and worst purchases.
The short answer
The Grain Trade Centre Office Key is the best value on the map: it is cheap, it has ten uses, and it is required by a mission chain you will run anyway.
The Main Guest Room Key returns the most per run but costs enough that it needs most of its ten uses to break even — and it sits in the most contested room on the map.
Reading the ROI figures
Every return figure on this site is a community-reported average, not a measured one, and none of them account for the loadout you lose when a run goes wrong. Treat the ROI ratio as a comparison between keys, not as a prediction of your own results.