Highest ROI home office purchase?

Get a high quality office chair from Steelcase or Herman Miller. A used Herman Miller Mira Aeron is better than any new office chair for of the same price. Replace the gas cylinder if it's worn. It's relative easy to do at home.

Sit/sand desk. Even if you don't use it in standing mode the ability to adjust the height easily is worth the price.

If you use a Macbook, get a monitor with complementary resolution.

Good lighting with dimmers.

Large monitor. I find it wildly difficult to do anything on a small screen.

Although maybe a standing desk beats it out.

Same as ever, POTS land line if it's a business office at home.

Sit/stand desk.