This one's easy. Buffy and Spike. Barring the reasons I posted yesterday about how that ship pretty much made my life today what it is, it's the one I'm most emotionally invested in, both in the throes and afterward. While Arthur was the reason I got sucked into Merlin, I was never invested in ships in that fandom as I was characters. I can multi-ship there because of that reason (though I will admit, Arthur/Percival might be one of the few to give Buffy/Spike a run for their money because those are my two favorite characters there).
But Buffy and Spike...man. My first real introduction to the potential there was The Gift. Craig made me watch the season 5 finale (eps 21 and 22 aired together in the UK) since there was a chance the show would get canceled. And I couldn't get past all my questions about why this vampire would be so broken by her death. The kind of loss and devotion they elicited is what dragged me into their ship, and while Spike stole my heart (over-identify much?), it didn't take long at all for me to feel as strongly about Buffy, or see all the ways they could be so good for each other. How can they not be my favorite?