Buffy and Angel! Their star-crossed love was amazing for each other, despite their inward and outward struggles! I think Buffy and Angel both always knew it in their hearts that the other one was the only person they could ever truly love! I liked Spike and Riley both, but I don't think she every really loved either of them. She obviously cared deeply for each of them, however.
Buffy and Angel forever!! How can you argue with perfect happiness? Plus, I quote Buffy: "I would have given up everything I had to be with--[Angel]. I loved him more than I will ever love anything in this life."