Fan favorite Sableye shares the distinction with Spiritomb and Eelektross of being the only three Pokemon who have no type weaknesses. A friendly Ghost-type, Sableye has beautiful gems for eyes, and like diamonds, Sableye is super tough (evidence: it eats rocks).
Sableye is known to be misunderstood within the Pokemon lore, since many people have superstitious prejudices against it, thinking it will steal their souls. On the contrary, Sableye are actually quite sweet despite their creepy resemblance to the Hopkinsville Goblin, an alien that was allegedly encountered multiple times in Kentucky during the 1950s.
Training tip: Sableye's well-rounded stats lean toward physical attack/defense, and it has plenty of Dark-type physical moves to choose from. Knock Off is a great move for Sableye, because while it only does light damage, it takes advantage of Sableye's STAB and attack stat, and it also removes the opponent's held item for the remainder of the battle.
Shuppet / Banette
Banette is pretty cute for a Pokemon that's possessed of pure hatred. The story goes that Banette originated as a plush doll that was thrown away by a child, and its feelings of hatred were so strong that it came to life to seek revenge on the child. Its Ruby Pokedex entry says it sticks pins into its own body to perform curses. Voodoo dolls are scary enough, but a sentient one that's so full of rage than it skewers itself is the stuff of nightmares. Does that mean Pokemon promotes voodoo magic against children?
Little Shuppet is almost as creepy, who thrives and feeds on feelings of jealous, vindictiveness and malice according to its Pokedex entries.
Training tip: Shuppet and Banette are two more Ghost-types that actually have a much stronger attack than special attack, but even then, Banette is strongest as a status inflictor. Will-o-Wisp is always a good choice to inflict burns, and Destiny Bond ensures at least one KO when your opponent KOs Banette.
Duskull / Dusclops / Dusknoir
The most recent of the pure Ghost-types (again, not counting Black and White) and the only pure Ghost type with three evolutionary stages (including B&W!), Duskull and Dusclops weren't the first Pokemon you'd expect to get a third evolution in generation four, since Dusclops is already such a strong Pokemon.
Duskull's grim reaper-esque design is pretty straightforward, but Dusclops is actually based on a combination of a mummy and a Japanese ghost called a chochinobake, which is a one-eyed ghost made from a lantern over 100 years old.
Training tip: The Duskull evolutionary family has strong stats in both defense and special defense, especially compared to other Ghost-types. It's worth teaching Pain Split, which takes the user's and the foe's HP and combines them, and then divides the total in half between the two, and can be learned through breeding or via move tutor in HeartGold/SoulSilver. Whether or not you want to trade Dusclops with a Reaper Cloth to evolve it into Dusknoir is up to you, but you do get a significant boost to physical attack by doing so.
Drifloon / Drifblim
Another Ghost-type with unique typing (actually, all dual Ghost-type families have unique typing except for Sableye and Spiritomb), Drifloon and Drifblim are dual Ghost/Flying types. Of the two, the diminutive Drifloon is actually described more eerily in its Pokedex entries, which say that it tries to drag children away to the underworld. HeartGold and SoulSilver's entries ominous state that "It is whispered that any child who mistakes Drifloon for a balloon and holds on to it could wind up missing." Eep!
Drifblim, on the other hand, seems much more benign, and is content to float around the skies aimlessly. At first it seemed a little odd to base a Pokemon on a balloon, but it actually fits well with the ethereal, ghostly theme.
Training tip: Drifblim has a weird combination of high HP and low defense stats which more or less cancel each other out. You can take advantage of its super high HP though by using a combination of Substitute and Baton Pass.