Like Charizard, Gengar has been a constant partner since the beginning. I didn't watch much of the show, especially after the first season, but one of my favorite episodes involved the characters exploring an abandoned mansion and being spooked by a comical Gastly, Haunter and Gengar.
Nevertheless, I continued on with him in my Johto journey, and he's made his niche as my best Sp. Attack sweeper. I've got a good array of coverage moves on him, including Shadow Ball, Sludge Bomb, Dazzling Gleam and Thunderbolt. His Levitate move keeps those pesky Ground moves away too.
It seems that a trend for this list involves me not liking these Pokemon's Mega Evolutions, and unfortunately holds true with Gengar. His shiny isn't great either, but then again, why mess with perfection?