could be either start by pullin spark plug and check it. aslo inspect your plug wire and make sure its not burnt or cracked anyware. and while at it i would check the compression ratio in that cylinder just for safe reasons.
I would check to see if it is a bad plug or wire.
If good, check to see if the coil pack is bad.
Check the injector
Do a leakdown test on the cylinder to see if the valve is not fully closing.