The difference between P and J can often cause the most conflict for me. However, which P and which J often has a lot of driving force behind this. It's interesting that this letter is the most diffucult to sort out for most, but tends to cause the most issues when there is a big difference. Personally, I tend to have the most "issues" with TP's. Like everything though, there are exceptions to this.
The dichotomy between P and J is really the largest, because it completely changes how information is taken in and processed. I mean you can look at the difference between INFP and INFJ. There are superficial similarites, but at their core they are very different. For one, those two types do not share any common cognitive functions. The way information is processed is very different. Just from a seemingly trivial change of P and J.