I'm not sure that Riley ever really loved Buffy as much as he saw her as someone with whom he could share his "secret identity" (and I'm almost certain that that is precisely how he thought of himself: a superhero defending the world from the "hostile subterraneans," while everyone thought he was just a mild mannered student adviser. I don't think it's any sort of coincidence that Xander, a comic book/action hero fan, would idolize him.) Much to his shock and dismay, Lois Lane was really Wonder Woman, and Riley just couldn't stand being only the love interest.
If anyone was obsessed with Buffy, it had to be Angel. From the moment Whistler recruited him, it was as though the Slayer would be completely helpless without his help and guidance. His, "I know what's best for you," attitude began about that time and only grew as the series progressed and grew even more rapidly once he got his own show. He never missed an opportunity to act in what he conceived as his friends' "best interests," whether his help was wanted or not. Angel wanted to take the helpless little Slayer under his wing and protect her from all of the nasties in the world.
Spike was unique in his feelings for Buffy in that he never wanted to protect her an never for a single moment doubted that she was completely capable of taking care of herself. I think Spike actually reveled in Buffy's skill and power, first as a worthy opponent, then later as someone who he could really love without holding back. Accept the fact: vampires are powerful, dangerous creatures, and Spike was exceptionally powerful, even for a vampire. He could share his passion with Drusilla, but her mental state left her more than a little deficient in the intellectual stimulation department. With Buffy, he found a physical match, someone with whom he never had to hold back either his strength or his passion, because she matched, or even surpassed him in both departments. Both the Slayer and the vampire met their perfect match, each in the other, a match that neither had ever known before, and probably never would again. Each has seen "the best and the worst" in the other, and regardless of what may come afterward, I doubt that we will ever see as perfect a match again.