For anyone that wants to diss Jay Leno or Dennis Rodman's involvement in WCW, let me just say this... those guys brought RATINGS to WCW when they did that. Particularly Jay Leno who even worked with TNT and promoted The Tonight Show out the ass and in turn, WCW got tons of promotion on NBC. N-B-FUCKING-C. That network is kind of a big deal, you know... and getting consistent promotion on their flagship late night program that shit tons of audience in your own target demographic watch is a motherfucking jackpot. So even though watching those matches on Youtube is pretty underwhelming, you can't honestly include them in a "worst of" list because they both served their purposes exponentially.
The best ever was unquestionably Cindy Lauper and the "Rock and Wrestling Connection". It was pretty much the whole reason WWE became as main stream as it is now. And if you don't agree, I'm sorry, but you're just plain wrong. Do some research and observe the cultural impact that had on the public perception of professional wrestling. There only other celebrity appearance that was even close to that was Mike Tyson (although even speaking as a tremendous fan of his 1998 run, Tyson had nothing on Cindy). His involvement was one of the most important things that ever happened to WWE, particularly in the 90's, and it was just awesome on every level. On a less-relevant note, yet one I immensely enjoyed... Bob Barker. I thought he was drop dead hilarious and I was frankly blown away with how well he meshed with WWE. He really didn't even stick out like a sore thumb like you'd think he would. Regis Philbin was another very good one.
And for worst, I know everyone is just trigger-happy to throw David Arquette's name out there because WCW put their title on him and even WWE still shits on WCW's former management for that, but it wasn't a bad idea to get him involved with their product. Especially considering he was starring in a movie that featured practically the entire WCW roster, so again, it was a two-way promotion going on there. And it worked... to an extent. Putting the belt on him was a bit much though.
For the real celebrity flops, I really can't look anywhere else but WWE, honestly. Pamela Anderson definitely springs to mind because she was just plain awful. Even reading her body language at the 95 Royal Rumble and the ensuing Wrestlemania, you could just tell the bitch didn't understand what she was supposed to be doing and clearly didn't want to be there. Mickey Rourke was also pretty bad, especially considering WWE didn't incorporate anything from his "Randy The Ram" character from "The Wrestler" or even attempt to go down that road to put some serious promotion into his movie. And that's really fucked up because the bluray version of "The Wrestler" has a really long interview featuring many wrestling legends, a few of whom were currently under contract with WWE and a few others that were still making sparatic appearances. And ALL of them made mention of WWE throughout the long as interview... and in return, WWE basically did nothing for Darron Arofnosky's movie and only really referred to Mickey Rourke as basically "some celebrity from that movie about wrestling". Very weak on WWE's part. And a lot of people praise William Shatner's involvement in the mid-90's, but frankly I didn't think he and Jerry Lawler had very good chemistry.