Spike & Willow
When Spike was vulnerable he kidnapped Willow to make Dru love him again. However, even though Drusilla was on his mind I know there was something there for our Will.
Again in Season 4, these two end up alone together, having a conversation. Ok, not a normal-type conversation but definately sparks. I mean, did you see that sexy look he gave her?!
Willow is a very emotional person, she likes to talk. While Spike is more of a do-er.
This relationship has never actually been a relationship but if it were, I believe it could work. Spike can be very loving, as we've seen with Drusilla. A relationship with Will could be good for him.
Mind you, I could definately see Spike with Willow's vampire self. Willow as a vampire reminds me of Drusilla, only more sadistic and commanding. She is like an adult Drusilla, or what Dru would be like if she wasn't insane.
Can you imagine what would have been the outcome if Spike had bitten Willow that night?! A new reign of terror in Sunnydale!!
Instead Spike got a chip and an unwilling alliance with the Slayer.
Spike and Willow will never happen. It is only in our imaginations that these two can have a relationship. But the evidence is there if the situation ever arises again....