I believe this was a thought-sparker, and I remember really liking it, so I thought I'd push it out again (though I can't remember for the life of me who the original asker was). Just thought it would do better here- it tends to bring up 'Lost golden days' talks, which are often better fitted for private conversations as they're rather exclusive.
I have way too many to choose just one as an ultimate, but one of my favorites is when a listee posed as Dutchy (Ivan Dudynsky). There were a lot of posers at that time, but he was particularly my favorite, as all of the sudden, there was a massive 'Dutchy!Love' moment on the NML, and various fans/fansites started popping up. Then it quieted and most of the sites were deserted after it was found that he was not so much he at all. It was all pretty hilarious, as was the reaction of the pre-Poser!Dutchy Dutchy fans, a few of whom were slightly less than thrilled. It was pretty amusing, and yes, I'm sure I believed it was the real Ivan Dudynsky at the time, too. I mean, why wouldn't an actor in his early thirties want to come hang out with a bunch of teenage and preteenage girls obsessed with a movie he did many years back in a chatroom in the middle of the afternoon or on those lonely Friday nights?
Also, when I talked to my first NML friend on the phone (I was SO excited, it was like I had just been introduced to the invention of the telephone), and she made me laugh while drinking a sprite and chewing gum, thus nearly choking me. Though, I actually think that memory has a moral in it somewhere, so it's not as amusing, but still, highly funny at the time.