Spike and Drusilla
Synopsis of Relationship: It could have started two ways. 1) British Guy gets turned into vamp. He then becomes Cool Vampire Guy and travels around Europe with Evil Vampre Guy. Evil Vampire Guy drives Sweet British Girl crazy, and then she becomes Insane Vampire Girl. They travel around Europe until Evil Vampire Guy bites a gypsy and gets his soul back. Brooding Vampire Guy goes elsewhere to brood. 2) Evil Vampire Guy makes Sweet British Girl go crazy. Then he turns her into Insane Vampire Girl. She turns British Guy into a vampire. But we do know this: Insane Vampire Girl and Cool Vampire Guy wreak havoc until they go to Sunnydale. Cool Vampire Guy does a ritual with Brooding Vampire Guy to restore Insane Vampire Girl to health. Slayer Girl, of course, intervenes and sends a beam crashing down on Cool Vampire Guy and Insane Vampire Girl. Cool Vampire Guy is stuck in a wheelchair; Insane Vampire Girl is fine. But then Brooding Vampire Guy turns into Evil Vampire guy (see Buffy and Angel) and lives with Insane Vampire Girl and Cool Vampire Guy until he goes to Hell. Insane Vampire Girl and Cool Vampire Guy (who is now out of the wheelchair) leave Sunnydale. Later Cool Vampire Guy comes back to Sunnydale to do a love spell on Insane Vampire Girl. It never gets done, so Cool Vampire Guy goes away to find her and torture her until she loves him again. Who knows what will happen in Season Four?
Why Spike and Drusilla Should Be Together
The Deadly Duo is personally my favorite couple. Spike loves Dru, and Dru loves Spike. They would kill for each other (no, wait, they're vampires, of course they'd kill for each other!) Spike would do anything for his Princess, and Drusilla would do anything for her Dark Prince. They communicate, and are quick to apologize. Now that's really a relationship.
Why Spike and Drusilla Shouldn't Be Together
Drusilla obviously has spurned Spike for collaborating with the Slayer, so why should he go back to her? She doesn't want him, and Spike made a fool of himself over her in Lovers' Walk. Dru, the last time we saw her, loved Angel. Besides, Spike belongs with Buffy (Willow, Cordelia, or whoever you want).