Even though Season 4 contains many of my favourite Muppet Show episodes and moments (Muppet Awards with Phyllis George; murder mystery with Liza Minelli; Gonzo's departure-and-return with Lola Folana; Dudley Moore's music machine; and great guest-star turns from John Denver, Kenny Rogers, Anne Murray, Doug Henning and Dyan Cannon), I have to give the nod, however slight, to Season 5.
It seemed like Jim Henson and company were determined to finish the show on a high and indeed they did, right down to the subtle but welcome tweaks at each episode's beginning (Pops welcoming the guest stars to the theatre; Statler and Waldorf's "Why do we always come here" verse in an extended opening theme song) and the end (Lips' trumpet part in the closing theme).
The writers and producers also came up with some wonderfully creative plotlines without sacrificing the basic formula (Carol Burnett blowing a gasket at Gonzo's dance marathon; "pirate" Glenda Jackson kidnapping the theatre and sailing it out to sea; Statler and Waldorf running the show, and botching it, during Hal Linden's visit; Johnny Cash's guest-star turn being simulcast on a country radio station; shrinking-and-growing Brooke Shields in the title role of Alice in Wonderland; Marty Feldman's twisted take on the Arabian Nights, ending with that amazing cameo by the Sesame Street cast).
So many other great guest-star moments too...Joan Baez, Linda Ronstadt, Paul Simon (allowing the Muppets, especially Gonzo, to have their way with his music catalogue), Roger Moore fending off Miss Piggy, Gene Kelly insisting he's not going to perform and then finally giving in, Buddy Rich and Animal in the Battle of the Drummers, Debbie Harry shifting between her Blondie hits and a sweet duet with Kermit on "Rainbow Connection," Loretta Swit playing temporarily-fired Piggy's role in Pigs In Space, and two of my favourite musical numbers of the whole series - Mac Davis serenading mermaid Piggy with "Baby Don't Get Hooked On Me" (the #1 song the day I was born!) and joining the Muppet band for "I Believe In Music." (And that episode ended with the Beaker clones playing the closing theme...too funny!!!)
Season 5 is also awesome because of episodes where the Muppets made the best of a bad situation (series writer Chris Langham's last-minute subbing for Richard Pryor) and delivered one of the funniest meta-Muppet sequences of all time when Gonzo, attempting to figure out puppetry in the Senor Wences episode, declared that "wiggling dolls is WEIRD!" before walking away, leaving Kermit to break the fourth wall and deliver the beautiful, classic line: "I didn't have the heart to tell him."
In terms of DVD releases, I'd consider Season 4 to be a victory at this point but I'd love it if Season 5 came out simultaneously - I know my wife and I will treasure both for years to come.