It's hard to pick a absolutely worst housemate of all time since many of them were just plain dull (Andy, Gos, Bubble) damp squibs (the guy from BB2 who was voted into the house to stir things up and didn't, the lap dancer from BB2 who turned out to be good hearted but didn't get down to anything nasty) total airheads (Jade) up themselves (Jason, Narinda) bunny boilers (Michelle and Emma from last year) or away with the fairies (Kitten). I didn't watch much of BB1 until late in the series so I can't say much about Nicola, but I'll mention her based on her reputation. I disliked Tim because he thought the puppet task was "childish". Elizabeth from BB2 was okay in my book until she told Helen to be careful about Paul, making Helen very upset. The housemate I disliked most out of all the series was Lisa from BB4 - she was so full of herself she seriously thought she had a strong chance of winning, although she came in around Week 6. Worst of all, she admitted that she didn't know if Osama Bin Laden or Saddam had been found because she never read the newspapers.
My Big Brother from hell would have Tim, Jade, Fredrico, Jason, Gos, Kitten, Emma, Michelle, Bubble, and Lisa.