I would have to say Key Lime Pie by Vapor Alley,it tasted really bad i ended up throwing it out.
Easy...CLOVE! Hands down the absolute worst flavor I ever had.
Not sure if it was the brand I tried (Dekang, I think) or the flavor in general. I used to love smoking clove cigarettes on occasion, as a special treat to myself. Was hoping to get a little of the same satisfaction from the e-juice but it nearly made me puke!

Any chocolate flavor I've ever tried has been gross. I can't find a good milk chocolate. They all taste like I just inhaled dry cocoa powder.
