Slimer (The Real Ghostbusters)
Promoted from bit-part in the movies to pet/mascot in the animated series, the green ghost quickly became a fan favourite. His habit of oozing ectoplasm didn't endear him to his human 'friends' but it was definitely entertaining to the kids watching. So much so in fact that the later series were even renamed Slimer and The Real Ghostbusters. Surprising cute for a thing that looks like he might just be made of snot.
Casper The Friendly Ghost
First appearing in a Paramount short way back in 1945, the pint-sized phantom stands out from his fellow ghosts in that he wants to befriend humans rather than scare them. Unfortunately for him, making friends with the living isn't as easy as he would like. The character has returned regularly over the years, most recently in 2007 with the CGI Casper's Scare School. In 1995 he even made it to the big screen in a live-action/CGI movie, which -disturbingly enough- finally explained how Casper died in the first place. Classic Casper shorts can now be seen on the DreamWorks Animation owned Retro Heroes YouTube channel.
The Ghost Of Christmas Yet to Come (Mickey's Christmas Carol)
Mickey's Christmas Carol is a surprisingly faithful retelling of Dicken's classic spooky festive tale. The most sinister part of the story has always been the visit of the imposing ghost of Christmas future. And much as with the later Muppets version, the third ghost is played absolutely straight, resulting in a sequence that could be genuinely scary for younger members of the audience.
The Headless Horseman (The Adventures of Icahabod and Mr Toad)
Disney's 1949 take on the short-story by Washington Irvine, The Legend of Sleep Hollow was originally intended to be released as the House of Mouse's next feature. It ended up running short though and was ultimately packaged with a long-gestating adaptation of The Wind In the Willows. In Irvine's original story it's heavily implied that the Horseman who torments Ichabod Crane may in fact be a human in disguise. However Disney's take is much more ambiguous (meaning it's totally fine for us to include him on this list!)
High Five Ghost (Regular Show)
Muscle Man's best bud is always on hand to offer a High-Five to punctuate his (normally Mom-based) jokes. In fact, with a hand growing out of his ghostly head, he seems to be uniquely suited to this purpose. Which begs the question, did he grow the extra appendage for this exact reason? Or was he already like that? Or, it could be that we're entirely over-thinking this... (surely not!)
No Face (Spirited Away)
In Japanese mythology the term Yokai covers everything from ghosts to demons, goblins and other supernatural beasties. However, we feel No Face is close enough to the western idea of a ghost to earn a place in this list. As perhaps the most memorable creation in Hayao Miyazaki's beloved Spirited Away, he is surely one strange cookie. A visitor to the bathhouse who becomes fixated on Chihiro/Sen, he causes a lot of hassle by eating several of the establishment's clients and staff-members. Ultimately though, he's not evil at all (just little misguided) and it's not hard to see why audiences fell in love with him.
The Funky Phantom
Hanna-Barbera were never ones to let a good idea go to waste so spent most of the 60s and 70s trying to recreate the success of Scooby Doo, with mixed results. In this case, the mystery solving teenagers were helped out by Mudsy the ghost of a soldier from American War of Independence (and his ghost cat) who they accidentally free from a haunted clock.
See also: 5 Fang-Tastic Animated Vampires