Giles/Joyce. Undoubtedly. And it
is canon because Jane Espenson said so. So nuh! Haha.
After them it's a toss between Willow/Tara and Xander/Anya. Willow/Tara because they were so sweetly romantic- it was wonderful and beautiful and all things are relationship should be (Willow's majiks addiction aside....). Xander/Anya were the personification of opposites attract- for example Xander hid behind witticisms whilst Anya used witticisms to be as blunt as Cordy, blunter even. They were the real deal and comical whilst being so.
But in all honest I love every single relationship on Buffy while it's happening. I love Bangel, I love Buffy/Riley, I love Spuffy. I love Willow/Oz but I also love Willow/Tara. Xander/Cordy, Xanya and I also have a soft spot for Buffy/Xander. Maybe this is just showing what a romantic I am. I'm a sucker for every single relationship. I even love Giles/Jenny despite being a total Joyles Purist.
There is one relationship in the Buffyverse that I completely and utterly hated and I'm sure many would agree:
Giles/Olivia
It didn't go, it didn't fit, it wasn't right. And c'mon if he really felt he needed to fill his mucho empty spare time with some lovin', couldn't he have given Joyce a call? Instead of some random British chick who well...just didn't FIT in Sunnydale or with Giles. She didn't belooooong!