I'm neither Spuffy, Bangel, Riffy or Xanfy but to me she had love for each of them in different ways.
Xander, Best Friend.
Riley, the opposite of Angel, allowed her to look at the sexual aspect of a relationship, and allowed her to love him for that.
Angel, ok i'll say it...first love. We always love our first love, and often hope that one day we can go back to that.
Spike. Yes Buffy loved him, she even told Angel so. 'He is in my heart.'
It was more then what she felt for Xander, less then what she felt for Angel, and different then what she felt for Riley.
She could never have had a lasting relationship with Spike, and i do think if he had left the cave with her they would have had a mature relationship from it, but honest to goodness, first thought in the morning last at night, can't eat cant sleep, and think kinda love....nope sorry.