Childhood's End had a big impact on me as a young reader, as did several volumes worth of Arthur C. Clarke stuff - mostly short story volumes (ah, Harry Purvis), although I have a very soft spot for The City and the Stars/Against the Fall of Night, which I had collected together in one volume and read in one fell swoop. ACC is one of a handful of sci-fi novelists who I blind bought, en masse, and by 'handful' I actually pretty much just mean him, H.G. Wells, and when I was a little older, PKD.
Again, short stories are handy here. You want to read Arthur C. Clarke's involvement in 2001? His novelization of the film is fine, but "The Sentinel" - the one Arthur C. Clarke story that most influenced the film - is better.
Clarke's pretty much the only one of the Big Three I really enjoyed as an author. Asimov was mostly only good for me for the puzzles in his Robot stories. What I basically enjoyed about those is, having established his rules, Asimov played with unintended consequences or possible results of those rules. That said, I recall the Foundation trilogy to be pretty turgid. It was Edward Gibbon without the wit or skill, as it were.
I, Robot and the first Foundation book are probably worth at least checking out, regardless, if you're a sci-fi fan.