BBC all the way, can't remember the last time that I watched the ITN news though have watched Sky occasionally just for a different perspective. I have heard that Channel 4 news is the best but haven't really watched it that often.
BBC World or CNN International. Not a great choice but at least for a few hours each day DWTV (Deutsche Welle TV) broadcasts in English too and their progs are much better.
Out here in the desert we can get both BBC World and Sky News. I always choose Sky as it has better (and much faster) sports news (it covers Championship matches, often with action highlights) and a better ticker service. If I'm working in the hotel room I usually have Sky News with the sound off.
Pasty makes a good point in relating the news output to newspapers, I guess that would make Channel 4 news something like the Guardian/Observer/Economist, appealing to the more intellectual viewer. Without trying to sound like a snob, I have always regarded ITV as tabloid TV, full of adverts and little real substance, I guess the news programme is put together in the same way.