Still working through the thread but I have a few I may have wheeled all the same ones out before, I don't get out much these days
Worst is definitely John Humphrys (on phone, not in person). Utter utter cunt. And this is long before the 'bantering' conversation about women's pay. Rude, surly, looking for a fight.
Dr somebody Joshi (jumped-up lifestyle guru/nutritionist/whatever, briefly in the spotlight with a newspaper column, book etc some years ago). Like a spoilt child. Phoned to speak to my boss, threw toys out of pram when I said she was in a lunch meeting and I'd get her to call him back. Phoned back several times, also phoned round the office and harassed colleague who was unfortunate enough to pick up the phone. Think he actually expected me to run into town and pull her out of her meeting to speak to him. 
On the upside:
Joan Bakewell, briefly while waiting for a lift. Beautifully dressed, coiffed and made up. Gracious. Made exemplary small talk and a light joke.
Graham Norton, briefly, in company. Surprisingly shy/overwhelmed at the attention. Twinkly. Genuinely funny and warm.
Paul Merton. Pleasant albeit shy. Lights up when talking about comedy.