For me it has to be BBC. There are so many reasons. I think the Beeb is still great value for money at £145.50 (colour) and don't agree with those who say it's far too expensive. Look at what you get for your money.
Advert-free viewing, news coverage and general programming, that by and large, is without any political agenda, despite what some would say. A radio network that is worth the money alone and a wonderful resource in the BBC website. That's just the top line without delving any deeper to individual departments and specific shows.
Don't get me wrong ITV isn't all bad, but it just feels so low-rent in a lot of its mainstream programming, particularly Saturday prime-time offerings. It's virtually given up on sitcoms and its sports coverage is fairly second rate too.
I'm sure there's plenty on here will disagree with these (personal) views and I now watch with interest to see how this will go (if it indeed goes anywhere)
I haven't commented on Sky as I feel as a non-terrestrial broadcaster and one that's paid for in addition to the licence fee, they're in a slightly different ballpark. Save to say that compared to the cost of the licence fee then they're bloody well expensive (I have a full subscription btw that weighs in at over £600 per year)