The question is about the epistle you yourself favor. This is not about which epistle you think is most important. Many of us would probably say Romans on that score. This is about which is your PERSONAL favorite, the one you enjoy most or the one that speaks to you most.
Some of you may consider Hebrews to have been written by Paul, but our assumption is that it is not. I don't want to create a dilemma for someone whose favorite epistle is Hebrews but yet does not believe Paul wrote it!
Please consider sharing why you favor the epistle that you do, or perhaps which set (pastorals, prison, etc.).