There are some very strong candidates among the 46 UK airports I've flown to/from, but sadly I haven't covered the rest of the world to the same degree and therefore can't give you the correct ranking. This ranking can of course be coloured of a situation you have experienced. Leeds-Bradford got such a negative vibe for me. My first flight to LBA was in the evening of 9 August 2006. I stayed the night over at the nearby Travelodge. The next day I had a short full breaky at the airport, but the main purpose was to get the key to the rental car I ordered. No smiles at all that day, only nervousness, but considering this was 10 August 2006, day zero for the transparent 1 litre plastic bag for liquids, my bad feeling about the airport was only natural. LBA lost this bad airport rating on my next visit, but that is a different story.