Most of the obvious choices have been listed already so instead of rattling off another list of 'penguin classics' i'll list some that haven't:
Cryptonomicon - Neal Stephenson, took me ages to get through but loved every minute of it.
HMS Ulysses - Alistair MacLean, one of the best war novels ever in my opinion.
The Outsiders -S.E.Hinton, surprised this hasn't been mentioned yet.
I'll also second Heart of Darkness and especially Watership Down, no other book has come as close to making me tear up as this, and I've no idea why, I mean it's about rabbits....RABBITS.
Oh, and I see your various childhood relics and raise you a BIGGLES. Hmm?? Anyone???