I wouldn't say any of the relationships were entirely healthy, but if we're putting them on a spectrum here, I would say Willow and Oz probably come closest. I realize Tara was supposed to be Willow's Great Love (capital G, capital L), but I thought she grew up in a healthier way when she was with Oz. Closeted sexuality notwithstanding, I think Willow grew for the better when she was with Oz: learning about self-esteem and forgiveness (two important components of a healthy relationship). Not that it was Tara's job to police her girlfriend or anything, but Willow did a whole lot of character regressing when she was with Tara. Her increased self-esteem was perverted into abject arrogance, and the things she seemed to have learned about forgiveness with Oz turned into her simply taking her loved ones for granted. All this to say, I think Oz and Willow really brought the good out of each other, so I'll have to go with them.
Runner up, Xander and Anya. In some ways echoing Willow and Oz, I think these two really grew up together, and became better individuals because of each other. Even if Anya reverted back to her demonic ways (but her heart was broken!), at no point during this reversion did she not feel remorse for her renewed vengeance acts. I like to think that being with Xander taught her something about love and humanity, and even though she no longer had powers, I think she grew to prefer humanity to demonhood because of her relationship with him. Likewise, I think Xander grew a lot being with Anya. Just like Oz helped Willow grow in self-confidence, Anya did the same for Xander. In one of his rare heartfelt speeches to her, Xander tells Anya that she makes him feel like he never has before: like a man. I think she genuinely cared for him and took the time to really know him. Despite that they constantly pointed out each others' flaws (often played for comedic effect), they also seemed to love each other for their unseen strengths. In these ways, I think they brought out a lot of good in one another. Even if they got off to a rocky, somewhat one-sided start, it's the fact that they grew to love each other that makes this relationship a good one. I wouldn't be lying if I said I was disappointed that they eventually broke up. And in true Whedon fashion, it had to be in some really dramatic way.
As for the unhealthiest (I know this isn't the question, but I can't resist answering):
Buffy and Spike. I just don't understand how people can forget how sick and twisted their interactions were. It astonishes me that there are hardcore Spuffy shippers out there who think they belonged together. Sex on the balcony, violent sex in the crumbling building, Buffybot, rape attempt, psychological abuse, need I go on? Perhaps he was somewhat reformed by the end of S7, but when they were actually "together"? Yikes.
Buffy and Riley. Widely recognized as one of the most boring couples in Buffyverse history, I consider this relationship unhealthy because Buffy was just using him (albeit in a different way than she was using Spike). In fact, I don't think there was a singly guy Buffy wasn't simply using post-Angel. Don't get me wrong. I think she cared about Riley in her own way, but I think ultimately she simply liked the idea of him rather than loving him at all. And even though, deep down, she knew she didn't love him, she held on to him because he was comfortable and easy and because she knew how much he loved her. When you realize you don't love the person you're with, you have the decency to break it off cleanly. Buffy did Riley no such favors, and the writers had to create some kind of ridiculous vampiress hooker storyline to break them up. I'm not saying Riley was great or anything (again, the boring thing), but namely I say this was unhealthy because it went on way too long considering how long the two characters knew it wasn't working out.
