I like the magnets on the end of an AA cell, and then placed in a long coil of bare copper wire.

But all of there are pretty neat and a good way to get interest started.

 

Homopolar motor courtesy of Michael Faraday, a while ago I posted the link to the brass bolt that unscrews itself.
Avalable on ebay for a couple of $'s 151865048522
Definition: https://en.wikipedia.org/wiki/Homopolar_motor
Max.

 

In the simplester motor the magnetic field is in the axis of rotation
The advantages that RichardO speaks regarding the Simplest Motor have a lot of merit.

The motor that RichardO posted doesn´t look like the magnet is polarized the same as the axis of rotation.

 

The motor that RichardO posted doesn´t look like the magnet is polarized the same as the axis of rotation.

It's not. I never said or implied it was.