KakaIru/IruKaka
Maybe I'll get flamed for this (although I doubt it because no one reads this journal), but I just don't get a Kakashi and Iruka pairing. Well, there's lots of pairings out there that are nonsensical, but I just don't understand the popularity of this one, which I think is second to Naruto and Sasuke. Unlike those two, Kakashi and Iruka really have nothing to back them up. They've interacted a total of, what, five or six times the whole series? How on earth can you possibly imagine them together? There's been no communication between them. Hell, even a Jiraiya/Orochimaru makes more sense than Kakashi/Iruka.
I think the origins of this stems from the very beginning of the series when there weren't that many adults (well, there still is very few in ratio to the teenagers, but yeah, more of them). Some people just either don't like writing or reading about pubescent boys and probably wanted to turn to something more mature. Okay, what people do we have? Well, even all the way up to volume five the only male shinobi over age twenty are Iruka, the Third, Ebisu, Kakashi, Zabuza, Gai, Ibiki, and Asuma, and the latter three are barely explored personality-wise. So let's just choose Kakashi since he's funny and Iruka because he's important to Naruto and call it a day. 'Cause you know we can't use Zabuza since he's with Haku.
That's really the only way I can imagine this happened.