Mice can be a problem or if you live some place where there is a lot of moisture that can also cause a problem. When you pulled the spark plug out of cyl 6 did you look inside to see if there is any residue and or anything that looks out of place compared to the others?
Really, it's just too hard to diagnose online with out actually being there to see and hear it in person. Your best bet, which you have already started on is by checking the easier things like the spark plug, spark plug boots, wires, electrical lines, etc.
As someone mentioned before, try swapping spark plugs, injectors from the other cylinders. It is very rare that a spark plug would misfire and most of the time it's the plug wires or injectors that are at fault, but if there are rodents chewing lines and nesting, that's never a good sign.