What is the best computer for teaching online?

There is no one specific answer.

At the school district I work for, most of the teachers use their school issued laptop alongside a second monitor. They use one display to present whatever they want and the other one to see their students.

I would look for:

At least an 8th generation Intel Core i5 CPU (or AMD Ryzen CPU)

At least 8 GB of RAM

At least 500 GB of SSD storage

Consider buying a second display, especially if you're going to be doing live teaching sessions.

A web cam if you're using a desktop.Though, they may be hard to come by currently since there is very high demand for them.

Ethernet connection over Wi-Fi.

For the most part, a laptop or desktop will work fine for teaching online. It mainly comes down to what works best for you. Desktops are great if you don't plan on moving your setup at all, or rarely. Laptops work well if you want to be mobile.

teaching what? molecular science? video editing? macaroni art?

each of those has a "best option" for computers.

"Best" depends on your budget. If you 

are going to be tele-conferencing with 

many students at once, then the more 

graphics power and bigger screens so 

you can see all the students would be 

likely the best setup. There is an eight 

monitor setup (see the link below), for 

around $3,200 (includes the monitors). 

I have seen even 14 monitor setups... 

again, it all depends on your budget. 

anything that is simple enough for you to use.  and, you'll definitely want a camera facing you [laptop has -- desktop, you add it on] for those Zoom meetings.  try some out at your favorite bricks and mortar store.  -- granddad

i think a desktop might be better for that