Probably not the absolute crappiest, but the most memorably crap.
I went to an experimental film party in a house in Hackney about 8 years ago. I'd been invited by my friend Varky who was the only person there that I knew. He'd been briefed on what to expect but had neglected to pass this on to me, so I thought I was going to a normal house party.
When I arrived, there were about 15 people in a darkened room watching each other's homemade films, and introducing them by saying stuff like "This is kind of a road movie, I suppose. About Neasdon." The films pretty much conformed to all the stereotypes of self-consciously arty film making. The host had also distributed a variety of electronic and acoustic instruments among the guests, who were expected to improvise an impromptu soundtrack (ie. make a total racket). Normally I'd be well up for this, but I was being conducted with a serious, somewhat po-faced attitude that took most of the fun out of it.
About half an hour after I arrived the film projector broke down, taking with it the whole purpose of the party, which was to show films. The guests weren't natural conversationalists, and were unprepared for the social demands suddenly placed upon them. The evening was slowly grinding to a halt, which wasn't good for me as I'd come all the way from Croydon.
Varky and I had been drinking liberally from the rum I'd brought with me, and by this point our drunkenness was starting to become apparent against the general backdrop of sobriety. Swinging into action, we commandeered the broken projector (which still functioned as a backlight) and organised an impromptu puppet show using 2D shapes we'd torn out of cereal packets taken from the host's kitchen. The details are hazy, but the show was a vampire story called The Thirst, and was presented in a shlockly, hammer horror fashion, probably with deliberately ironic intent vis-a-vis the party's high-cultural atmosphere. I vividly remember the host's demeanor as we rampaged round his flat preparing this spectacle - somewhere between peevishness at the liberties we were taking with his possessions, apprehension at the embarrassment we might inflict on him, resignation that this might be the only way to save the party from a total washout, and I think some small amount genuine curiousity at what was about to unfold.
I'm biased of course, but in my opinion the show was a roaring success. I narrated, Varky excelled in the role of puppetmaster, and the audience raised thier game to produce a soundtrack of genuine atonal menace. To be fair, everyone lightened up a bit after that and I had some interesting conversations. During the interminable nightbus journey home, though (it took about 3 hours if I recall) Varky and I allowed creative tension to get the better of us and had a massive drunken argument about nothing, which resulted in us not speaking for months. I choose my parties more carefully now, and I haven't had to deploy the spontaneous vampire puppet show icebreaker since.