Plants where your eyes rest
The measured effects of indoor plants on attention and stress are small, and they keep replicating. The tradition never waited for the studies: a living thing in the sightline reads as ease.
What the studies find
Office and classroom trials report modest gains in attention and modest drops in self-reported stress with plants present. The effects are small, they replicate more often than not, and they are about being able to see greenery rather than about the plant itself.
Air cleaning is the myth
The NASA experiment everyone cites used sealed chambers. Scaled to a real room with real air exchange, you would need something like ten to a thousand plants per square metre to matter. Keep plants because a room with something alive in it feels different, and open a window for the air.
Where to put them
In the sightline from where you sit, not in the corner you pass. The measured effect is a visual one, so a plant behind you is a plant you are watering for someone else.
What survives neglect
Pothos, snake plant, ZZ, cast iron plant. A dead plant in the corner is worse than no plant, and the tradition agrees: dried, sick or plastic greenery is specifically discouraged.