One of my absolute favorite pairings is that of Hatake Kakashi and Umino Iruka from Naruto. They are, of course, entirely a fan pairing, not canon, but somehow they grabbed me and refuse to let go. I enjoy a wide range of BL pairings, but KakaIru may just be my #1. I even started doing my own scanlations because of a KakaIru doujinshi, so you could actually say this pairing changed my life, hahaha. Here is an image (which I colored myself) from the aforementioned doujinshi:
I like them because they fill the holes in each other's characters. Kakashi is the consummate soldier in a world full of them, and Iruka is the consummate human being, yet they share core values that bind them together. I think such individuals would teach and fulfill and heal each other their whole lives through. This pairing is also flexible for any genre of fanwork, too. They can be tooth-rottingly sweet or extremely dark, strictly platonic or overtly romantic, side-splittingly funny or grimly serious, and it always works.