Cloak (and his partner Dagger) have a lot of marks against them. Their origin story is one cliché and contrivance stacked up on top of another, in the Mighty Marvel fashion. Ty Johnson fled his poor neighborhood in Boston after his speech impediment accidentally caused his best friend's death (which would give him and Black Bolt something to talk about if not for, well, you know). He ends up in New York City, where he befriends the naïve, wealthy, beautiful blond runaway Tandy Bowen, and the pair was kidnapped and used to test a new kind of synthetic heroin, which triggered their latent mutant genes, and they developed opposite yet symbiotic powers. How conveeeeenient.
But, despite their origins, Cloak and Dagger have always had a special place in my heart. I'm not sure where I first encountered them, though I suspect it was as guest stars in a Spider-Man comic, but they grabbed hold of my youthful imagination. I mean, sure, they're kind of a package deal, but Cloak's easily the more interesting of the two. After all, Dagger's still pretty normal, and she's far more well-adjusted. She even works well solo. Cloak, on the other hand, is troubled and troubling. He speaks in the sort of Byronic prose you expect from someone like Jason Blood, always talking about the darkness which threatens to consume him and everyone around him. And he isn't kidding; Dagger's still more or less human, but Cloak is a living conduit to the Darkforce Dimension. Most of the time, he lacks a corporeal body, and both the darkness within him and his inner demons are quite literal. It was long before I ever picked up an issue of Sandman that I saw Cloak talking in black word bubbles with white text, and just knew that it meant he had the sort of deep, intimidating voice that would send chills down James Earl Jones's spine.
The same goes for personality. Dagger's pretty cheerful, pretty normal, But Cloak is like a walking mass of passions. He's quick-tempered, protective, and moody to the extreme. And that goes double if you're talking about Dagger. Ty and Tandy's relationship is one of the most unique in all of comics. Cloak needs Dagger, but feels guilty for that; he's very protective of her in an older brother sort of way, but he's also distant and inaccessible, probably because he feels the need to protect her from himself. And they cap it all off with this boiling sexual tension, though they never (to my knowledge) have actually been romantic. And that's part of what sets this relationship apart; Harry and Sally haven't slept together yet.
What it all comes down to is this: Cloak and Dagger have the most well-developed Beauty and the Beast relationship in comics. And like nearly every Beauty and the Beast story, the Beast is far more intriguing.