You stated you have checked the wires, distributor cap etc. I have had carbon tracks on coil towers and inside distributor caps, they are hard to see and inside the cap they can prevent firing on a single cylinder. I have had bad wires that looked good and actually checked good with a ohm meter. Checking the spark by pulling the wire and watching the spark jump to ground does not always mean it will fire inside the cylinder under compression.
If your other plugs look good, if it were me, I would replace the distributor cap, rotor and wires with new ones. If that does not fix the problem then I would look else where. Good luck.