The worst live act I ever saw was Jesse Camp's band. he was an MTV VJ in the 90's. I saw him open for Alice Cooper in Atlantic City in 1999. It was the absolute worst . A disgrace.
..and then followed by Faster Pussycat.
Worst live act...hmmm...
I fortunately dodged the Faster Pussycat bullet.
My initial reaction was to say either Vinnie Vincent Invasion (opening for Maiden on the SIT tour in Feb. 1987) or LA Guns (again opening for Maiden on the SSOASS tour in June 1988). I flipped off Mark Slaughter, and he saw me and returned the favor (we had a real connection). I can't actually remember LA Guns, so it's possible that we didn't go to inside the building until they were done.
But then I remembered that I was subjected to Tori Amos in August 1994. I "had to" go because my girlfriend (now wife) liked her. AWFUL show...at least for me. Not only could I not stand her, but she exacerbated the headache that I started the night with.
The real answer is probably some crappy Sunset Strip band that no one's ever heard of, but I don't remember any of them by name, so I'm going with Tori.