Why is a toroid most often used as the top load on a Tesla coil?
It mainly has to do with curvature. The electric field is more concentrated at places that have higher curvature. A sphere has the same curvature everywhere, so there's no preferred place for a streamer or arc to start. A torus has the greatest curvature at its outer rim, so streamers/arcs are more likely to start there. If you make the minor diameter of the torus smaller than the diameter of the secondary coil, it helps make sure that the arcs come from there and not directly from the coil itself, which could damage its insulation.