An episode of It's Always Sunny that had it all - completely over the top moments of the gang ripping on Dee's resemblance to a bird (complete with remembering her as an actual ostrich), horrifying implications (including the possibility that Dennis knocked up Dee), the Rashomon-esque narrative structure, and McPoyles being extremely McPoyle-y.
Parks and Rec has a solid history with Halloween, but it's Halloween Surprise that takes the cake (apologies to all you Greg Pikitis/Mark Brendanawicz fans out there). Costume-wise, Leslie's "We Can Do It!" costume is maybe the most perfect thing that has ever happened on television up to that point - until the final scene where Ben gave Leslie a surprise proposal and the apartment of her dreams. It's almost weird to remember how soap opera-ish Parks and Rec was at times, and even weirder to remember how well it worked.Also, Ron and Andy trick-or-treating is pretty great.
While Modern Family has done a fair number of solid Halloween entries since, the original episode set the foundation for a lot of the great bits the show would play with later on - particularly Claire's love of truly terrifying decorations. Add to that Mitchell's desperate attempts to get out of the Spider-Man costume he wore to work, and you end up with a solid episode of a show it feels like no one has spoken of in 4 years.
How I Met Your Mother is a lot like Lost - the early seasons were great, full of ideas and promise, exploring lots of new concepts while teasing out long mysteries. But eventually the characters became grating, the mysteries got drawn out, and the answers were never very satisfying. But that's why Slutty Pumpkin is one of the all-time great episodes - the idea of Ted repeatedly having to dress as his once-topical "Hanging Chad" costume year after year in the vain hope that he'd meet his dreamgirl once again, the allusions to Charlie Brown, and the mystery of who the slutty pumpkin truly was all made for an unbelievably solid episode.And then the episode like 5 years later when they revealed the Slutty Pumpkin was Katie Holmes? Let's just pretend that never happened.