For me, Tara and Willow. In my opinion, that pairing was a trend setter/bold statement in the sense that, a lesbian couple's relationship could be a sweet, endearing and beautiful thing. Just like a hetero couple.
Honestly, the pairing was so good that once I got past the 'Oh two girls kissing and crap, I'm getting excited." (which happened very quickly), I wanted to see the two blossom together more than anyone else. (Anya and Xander's relationship is something I can Now laugh at and relate to)
Also, to me, Oz and Willow wasn't anything special. He's locked up a once/few times a night and they are a normal hetero couple. Willow and Tara seemed to connect very deeply and their joint magic practices gives them a even deeper understanding and connection.
Again, my opinion....plus I hate Oz's character, so there's that.