OK, OK, I know I've already said Dangel, which I love, but recent events have persuaded me to change my decision.
Now, normally, I would go off about how much I love Spuffy, and how Spuffy is the best couple on t.v. (Because, IMO, it is), but this time I think I'll go a bit differently. Although Spuffy could easily be described as the most detailed, layered, deep, and passionate couple on the show, I kinda feel as though their time is done. True, I still hope for them to eventually have a confrontation, preferable leading to passionate kissing 😀, but if not, that's OK too.
So I'll choose Spangel. These guys have been friends since the beginning. They're relationship was... challenging, and often extremely competitive, but they always stayed on good and complimentary terms with each other. Angelus taught Spike the ways of vampirism, and Spike eagerly learned, both pleased for a male companion. Continuing forward to the 1940's, they were stranded on a ship together. Even if he didn't want to, Spike always did what Angel asked him to, which was very cute. 🙂 Although Angel could've, he never killed Spike, even though Spike had killed some of the marines.
Moving onto School Hard, Spike knew Angel well enough to figure out he had a soul. When Angel lost his soul, he soon went back to live with Spike and Dru, which gave him and handicapped Spike plenty to bicker over. Skipping further along, we get to my favorite Buffyverse season ever: AtS S5.
In this season is the main reasons I love Spangel. As shown at the end of Damaged, they both really understand each other, and live extraordinarily similar life paths. They'll quarrel over anything (A Hole In The World), which turns both of them into the funniest characters on the show ("You just want it to be the way you want it to be." "It's not about what I want!"). But as soon as something important happens, they instantly start working together, easily able to communicate with each other without speaking. Spike is definitely able to bring more emotion out of Angel than anyone else, considering that Angel is a rather broody character. They both remember everything from their pasts, and are comfortable talking with each other about anything. They seem to be fine with holding hands (And judging by context they've done it before) and have apparently been intimate before. Also, it's obvious that they really care about each other throughout it all, and Spike actually said in Angel S6 that he cares about Angel.
In conclusion, I'd say they have the best and most real mutualistic love/ hate/ frenemy relationship on the show. 😀