Beyond your plug being properly gapped, and beyond your points being set and having clean, flat mating contacts, a good ignition condenser is vital as far as getting a strong spark.. a coil alone will (might) spark the plug but the condenser magnifies it.
The magnetic field in the coil collapses when the points open. This collapse causes a burst of electricity.. this high voltage electrical impulse goes through the spark plug wire and then jumps over the spark plug's gap and makes a visible spark. However, the wires in the coil are very thin and very long, and that path has a high resistance.
A condenser acts like a low resistance path for the coil's electricity and speeds up the collapse of the magnetic field. The faster this happens, the hotter the spark.
If your spark plug is trying to operate under conditions that might cause it to misfire (rich fuel mix, too much oil in the mix, some type of ignition weakness, etc) a good condenser is even more important.