Sounds like a bad secondary coil.Here is why I believe this.The coil pack has 3 primaries and 3 secondary terminals.The primaries are 3 of the pins off the main harness (3 yellow wires usually).That is your 12V input pulse signal from the PCM,one to each coil.The secondaries are your plug wire terminals.There are 2 secondary terminals to one coil.Now,when a coil fires for say..cylinder 2,a lower voltage spark is also applied to cylinder 6. 2 and 6 are on the same coil.1 and 5 on another,3 and 4 on the third.
So if all but one plug is firing and the adjacent secondary plug IS firing then you probably got water in the coil pack somehow and it's internally arcing instead of though the plug terminal.Get an ohmmeter,and with the engine OFF,test the resistance across all 3 coils.Look at the attached image below.Now,test across terminals 1 and 5,then test across 2 and 6 then across 3 and 4.You should have a reading between 9k Ohms to 15k Ohms.If you get a good reading on the coil with the non-firing terminal then you probably got water inside it.
a new coil will run you around $60 or so at your local auto parts store.