AB Maxx Brazil - the first one I smoked tasted like I burned the entire cigar in my ashtray, and then threw it in my mouth. I just did not enjoy it at all. The second one I smoked, months later, was much better.
5 Vegas Gold maduro - this has been well documented. I don't even know how many times I relit this thing. Tasted horrible by the end of the cigar. I actually enjoy their classic, gold, and high primings lines. Granted, I smoke most of those as yard-gars, but still.
Acid Kuba Kuba - Ick.
My brother in law gave me a sampler of cigars for Christmas a few years back. One was a Big Butt. I let it sit in my humidor for two or three years because I was so afraid to smoke it. That thing was the most horrible, vile thing I have ever tasted. I don't know how I finished it. I don't know why I finished it.