Difficult to name a specific "worst" HM - there's been quite a lot of really bad ones!
My most hated HM is definitely Kenneth Tong, but that's almost entirely because of what he did outside the house: he wasn't as bad inside (and wasn't there for very long anyway).
Alexandra (BB9) was spectacularly awful, but I hold the producers responsible for that as she should never have been put in the house in the first place.
I've long said that Charley (BB8) was the only HM that actually made me switch off, although a few others (including BB10 Charlie mentioned in anther thread ) came close. Charley wasn't just an irritant though: she could be a right nasty piece of work too - something which (going by recent threads I've seen on DS) people often forget now.
Of the others, most have already been mentioned here:
Grace (BB7) - probably the first HM to get me screaming at the TV. Continued her bizarre vendetta against Susie outside the house until threatened with legal action.
Sylvia (BB9) - Alexandra's 2nd-in-command and often forgotten about.
Bea (BB10) - Her fake crying as she tried to get Marcus into trouble for "bullying" her still sticks in the mind.
Dave (BB11) - The fake monk. Another one most annoying for what he got up to outside the house, and the fact that the press were so late in publicizing his activities. Came 2nd, which would annoy me more if it hadn't been so obviously due of BB manipulation.
Jay (BB12) - Frozen excrement boy. Unquestionably the most disgusting HM ever.
Connor (BB13) - Another "screaming at the TV" HM. Nasty piece of work, and the thought of him walking off with what was essentially a ÂĢ50 grand bribe from the producers still sticks in the craw.
...and of course loads of others which I've thankfully blanked from my mind for the time-being!