The first time i really noticed Superman and could see that he was someone truly special was All Star Superman #9.
Superman returns from his trip from Bizarro world and sees that two Kryptonians:Bar-El and Lilo have arrived on Earth. Superman hopes they could become Earth's next protectors. But his hopes are dashed. They serve as antagonists in that issue.
But once they find that they are dying, Kal actually helps them. They are really satisfied by Kal's dealings and his compassion. I have not seen something like this in any other superhero before.