Simple and traditional, for great old-time tone. Here's a well-made example of one of the earliest banjo tone ring designs.
The round rod tone ring is an unplated brass hoop, 0.275" round in cross-section, that simply rests on top of the banjo's wood rim. Uncomplicated yes, but the workmanship is impressive: it's nicely fabricated and lays flat, allowing more consistent banjo head tensioning. A great choice for a traditional old-time banjo sound, in 11" and 12" diameters.
A high crown or medium crown banjo head is usually required for this tone ring.