Well, I've been reading SF since 1962, and possibly earlier. (I may have mentioned this somewhere else that you've read, but the first SF book I can remember buying with my own money is the 1962 edition of James Schmitz' Agent of Vega. Which almost certainly means I'd actually been reading and maybe buying the stuff even earlier, but don't actually remember it.) Starting to read SF before the age of seven, being a fast and voracious reader by nature (with a parent willing to subsidise one's addiction), and having little social life as a child gives one a head start on lists of this nature.

And yes, you are absolutely right, T.H. White's Once and Future King should be there on the list.

Interesting that you mention Samuel Vimes - the Discworld book I was given is Night Watch and it's largely because of my liking for Vimes that I've decided to go back and read the series. I shall bear in mind your comment about Pratchett maturing as I read the earlier works.