It depends on what you mean with "the biggest hint".
- If you are asking about the first time I thought it was obviously the official couple, it was in chapter 3: Naruto introduces a new girl saying he likes her; that girl starts screaming him when she sees him; later she decides trying to treat him better.
- If you are asking obvious hints in general:
- Naruto goes to fight Zabuza. Sakura blushes and thinks "What is this I am feeling? Since when Naruto...?"
- After a mission Sakura is the one is carrying a limping Naruto. Later Konohamaru asks if she is her girlfriend
- Sakura ready to give up in the written test
- Iruka telling Naruto and Sakura are like heaven and earth, two halves of an everything
- Naruto atop of Gamabunta vows protecting Sakura... at all costs! (and Gamakichi tells his father Sakura is his Naruto's girlfriend)
- Naruto returns with Jiraiya and Sakura blushes wondering how much he has matured. Then he acts pervert, she hits him and yells he has stirred feelings inside her for nothing.
- After hearing what happens to the jinchuuriki, Sakura attempts teeling Naruto she is crying over him.
- Sakura sees Naruto turned into four-tailed fox and then she clutches her heart like if in pain, muttering "Naruto..."
- Yamato's words: "Sakura, in reality you..."
- Shortly after that scene, Sakura wants giving up the hunt for Sasuke for the sake of Naruto
- The feeding scene
- Sakura healing Naruto's arm and he telling he loves being healed by her (she blushed? I do not remember).
- Pain obliterates Konoha and Sakura pleads Naruto returns
- The hug scene
- Sai's words to Naruto and later to Sakura
- "Naruto, is that your girlfriend?" "Well, sorts of..." "Take care of my son."
Those are the biggest I can think of.