Obvious answer is obvious. ;p
I've been very dedicated to Dean/Castiel for the last couple of years now. This is the first pairing I've shipped in this particular fandom actually, it started towards the beginning of the fourth season of Supernatural and it hasn't let up, and I don't see it letting up anytime soon.
It's kind of difficult to really explain and pinpoint my love for this pairing, because it's a mixture of many contributing factors. Now I could go into the canonical eyefucking and endless personal space invading and the wall!slamming and the subtextual innuendos, but honestly it runs much deeper than that. There are other people that have manifestos that describe it in thorough detail, and I suggest reading tracy's explanation about her reasons which I pretty much mimic everything she says there. Though in my own personal shorthand version, it's the chemistry and connection they both share with each other that is completely undeniable; it's how they complete one another without really realizing it. There's a level of understanding and trust between them as the seasons continued, and they have some similarities that mirror each other which makes them even more perfect. I just, I really love the human/angel dynamic, which brings another thing that I love about their relationship on the show, being is how they're each other's first friend and confidant outside of their families. The kind of friendship that goes out from their normal realms is fascinating to me, especially when they're both learning things from each other, learning and growing from their experiences together. Something about their friendship, their connection just clicks with me from that alone. Plus, there's a certain salvation theme between them, and how they need each other through these rough patches they have during these last two seasons. It's just perfect, so perfect between them.
Ah, I could go on and on, honestly. I really need to write my own manifesto, heh. I just love Dean/Castiel very much, it's truly is my One True Pairing because I don't really ship anyone else on the show as passionately as I do with these two. I can't help but love everything about them, their friendship, their dynamic, the angst, the USTing, just everything. ♥ I also have a feeling that if the show were on HBO or Showtime, or if Kripke and Co. had the courage and balls to have homosexuality featured on the show (aside from Crowley, that is) Dean/Castiel would've already been established by now. Truth, my bbs.
Runners Up: Six/Gaius (Battlestar Galactica) with their crazy twisted yet adorable love, Helo/Sharon (Battlestar Galactica) and their powerful love that keeps them together through thick and thin, the complete adorableness of Richard/Kahlan (Legend of the Seeker) where I cannot help but smile whenever I see them, Victor/Sierra (Dollhouse) who just bring out the cuteness factor in such a bleak world, and the love/hate UST-filled relationship of Faith/Buffy (Buffy the Vampire Slayer).
Masterlist of days here.