what is positive aspect of huckleberry finn?

Well, I don't think I can provide ALL the positive aspects of Huckleberry Finn but I'll give you five:

1 - A concern for other people: Huck is a goodhearted fellow who hates to disappoint people. Circumstances force him to do so throughout the novel but it's always matched with a serious guilt. Leaving the Widow behind and taking Jim makes him feel bad, while the thought of betraying Jim also makes him feel bad.

2 - Adaptability: Huck is able to roll with what comes. He's able to handle being "civilized" even if he hates it, and he can equally accustom himself to the different societies that his trip forces him into to - the warring clans, dressing as a girl, living with Tom's aunt, etc.

3 - Clever: Huck constantly shows his ability to solve problems through cleverness. He escapes his dad with a clever ruse, he is willing to dress as a girl (it fails but he tries), etc.

4 - Loyalty: Huck considers his loyalties unbreakable. Even when he doubts Tom's plans for the long faux-rescue of Jim, he won't doubt it. He trusts his friend's cunning. And when push comes to shove, he shows a great loyalty to Jim.

5 - Observation: Huck, as narrator, shows a great aptitude for noticing behavior and asking questions about them. For a novel whose main purpose is to question injustices and how they shape youth and society, a narrator like Huck is valuable since he doesn't just accept everything as it is. Even when he acquiesces to social forces, he questions them, notices the effects of everything.