There are many conflicts in this novel, so I believe it is a matter of opinion as to which is the main conflict. Some would argue that the main conflict is Huck vs society, a "pure" child struggling against an evil society that is trying to civilize him while he wants to remain a free spirit. Others believe that the main conflict of the novel is Huck against himself, struggling to figure out what he should do about Jim - turn him in, as society dictates, or hide him, as his conscience dictates. A third view is that the main conflict supersedes the characters and is a struggle between right and wrong or good and evil. Good arguments can be made to support all of these views as well as others. If you are writing about this, you need to decide for yourself what conflict you believe is the main one before you can decide which conflict or conflicts are secondary. What makes this novel so rich is that there is ample evidence within the narrative to support many differing views. I have had students write about this subject from totally different viewpoints and I have agreed with them all!
There is a lot of good information about this novel right here on enotes that can help you decide on what the conflicts are.