I've read 10 different Conrad books.  They're all great.
Is Heart of of Darkness your favourite Conrad book? 
Yes.  Second favorite is Lord Jim.
It seems you enjoy novels that have a flawed hero and are quite tragic! Do you enjoy more serious novels?
I think almost all Conrad books are very serious.  And I enjoy that.  Deep, dark, philosophical/psychological books are my favorite.
Conrad seems to have led a really interesting life and it makes his books much more believable. He's great at showing life at sea and in the tropics. It's a shame all his works aren't more popular.
Yes, I agree about the sea and the tropics.  I feel like I'm almost there.
Have you read all of his books? Or do you still have more to enjoy?