The thread started out as a question of who loved Buffy the most, but since it has gotten around to which vampire she loved the most, I've just gotta toss in my opinion. So here goes.
Buffy loved Angel. No one could ever doubt that. And I fully believe that she WANTED to love him the most, always. That was the main reason that she denied her real feelings for Spike for so long before finally acknowledging them. She wanted Angel to be her one and only, her shining, if somewhat tarnished, prince. But in the end, it wasn't Angel who she chose to fight by her side, and it wasn't Angel who she spent what could very well have been her last night on earth with. I think that she finally began to realize there in the Guardian's crypt, after her fight with Caleb, that it was Spike who gave her back the fire (referring here to Season 6 Buffy song, "Walk Through The Fire") She left Angel there in that cemetary and went back home to share Spike's bed. She could just as easily spent the night with Angel. There were obviously plenty of vacant houses available. He could have "just held her," the same as Spike did the night before. They had done it before, after they knew about the curse on Angel's soul. They could have comforted each other one more time before the went their separate ways once more. She had a choice of who she would spend her last few private hours with, and she chose Spike. For me, that settles any doubt as to who the Slayer loved the most.
So, Spike loved Buffy the most, and just by coincidence, she loved him the most, too. It just took her quite awhile to admit it.