I firmly believe that Hunger Games is a rip off of Battle Royale, but to really see the point you have to look at Battle Royale as a franchise, not strictly a book, a manga or a film.
Look at the basic scenario of the Hunger Games - A tyranical government institutes a law that a game must be held where teens must kill each other. The game is held as entertainment for the high ups, and to keep the lower classes oppressed. It keeps the people in line. A teenager enters the game, manages to defeat the game instead of simply winning it. This person then goes on to inspire and head a rebellion against the government that put them into the game in the first place.
From one piece of the Battle Royale Franchise you will not get this full story.
-From the novel you get the initial concept of the game of teens stuck in a kill or be killed game. The novel also gives us the most complete portrait of the world in which the games occur, and how the games occur in order to inspire fear and mistrust among the populous so they can not band together and rise up. We also get the ending where the two primary characters (though BR does give you insight into almost everyone and has several strong supporting characters) both survive because they defeated the game (through the help of a supporting character)
-The manga takes the game and makes it a public spectacle. Things are done for ratings.
- The films give us the story of what comes after. They show Shuya (the main protagonist of the story) going on to inspire and lead the rebellion against the government that started and continues the games. We see Shuya, unknown to himself, put back into a game that has the sole purpose of killing him.
Combine them, and you have the entire premise of Hunger Games.
The primary differences that are really viewable are primarily the stories point of view, and the general tone (which had to be changed in order for Collins to tell the story for a teen audience), and it must be conceded that the last book of the series covers a "chapter of the story" that the Battle Royale franchise has never extended itself too.
It's bothersome that Collins has insisted time and time again that she had never heard of Battle Royale until after she wrote her book. It seems improbable to me that in researching material for her book, in looking up things like game theory or even kill or be killed games that Collins didn't once encounter Battle Royale. It also seems rather improbable to me that in summarizing the main plots of the books, you hit so many of the same points. It's absolutely true that you can not copyright an idea, and I don't think anyone would fault Collins for saying "I had an idea, and found this other story covered it... I looked into it and then decided that what I wanted to tell was tonally different enough from the original that i still wanted to pursue it". It's the sort of thing that happens all the time - Grant Morrison publishes The Invisibles, which has large portions ripped off by the Matrix, which then has large portions ripped off by subsequent works.
While there are decided comparisons to make between The Running Man and Hunger Games, they're only really evident when you get into the general sense of a kill or be killed game created for entertainment.
The moment you hold up the concepts of Battle Royale to the Hunger Games, you really can see a point for point comparison between the two stories (as I have enumerated this before I won't waste time and space with it again). What seems so unusual is that not only have fans called out Collins on the comparisons between her story and BR, but people like Stephen King felt the need to mention the similarities, as did the New York Times.
What seems most bizarre about Collins claims of her storys origin is that she has specifically left out any sort of pop culture influence. She continues to cite mythic roots to her story which don't seem to be evident on any real observation. Does Collins even cite material like Running Man or The Lottery? No.
It would be much, much more believable to think that Collins has invented a very similar story to Battle Royale on her own if she had cited things that seemed to influence Koshun Takami in the creation of BR, or even other material that had played with the idea before. From there it is simple extrapolation to come up with a similiar idea to what's been done because the same influences contribute toward it. However, Collins continues to flat out deny any popular influence, as if Hunger Games developed in a bubble that had no outside influence.
I would also argue that despite protestations that not everyone had heard of Battle Royale, it must be remembered that
A) Susanne Collins worked as a television writer before she started writing YA books. More often than not people working in the entertainment industry will hear and see films that the average Joe on the Street will never have heard of. It must also be pointed out that IMDB lists the production center for Collins series Generation O! as Canada, where BR was available for purchase.
B) As someone working primarily in childrens entertainment during the late 90's early 00's Collins would definitely had hear ear to the news of the fall out after Columbine. BR the novel was released in 99 and the film was released in Japan in 2000. One of the primary reason that there was not a US release was the distributors were scared that should another act of school violence occur after the films release, they would be implicated. It seems absurd now, but remember that after Columbine Washington was casting a stern eye toward the entertainment industry (It has to be noted the other primary factor was that the Japanese production company wanted it to have a wide American release, while the interested US distributors believed a subtitled film would never make money in wide release)
C) As both a writer of television and a writer of fantasy books, if Collins was ever invited to attend any sort of Science Fiction or general entertainment convention, copies of Battle Royale, as well as Battle Royale merchandise has been widely available at conventions since shortly after it's Japanese release.
The point is that Susanne Collins had ample opportunities to have come on contact with Battle Royale as either a novel or a film well before 2008. It must also be argued that as Collins did research on people put under the circumstances of a BR or HG type game, any half hearted internet search would have yielded copious pages talking about or referring to BR. Remember that, although the film has never officially been released in the US, both the novel and the mange have been widely available since at least 2003 (when I bought my copy).
To bring it all back....
-Collins has ample opportunities to come in contact with BR prior to writing the Hunger Games. -Collins fails to acknowledge ANY story which involved similar concepts as influencing her ideas, which seems bizarre considering that there are multiple stories which obviously cover similar ground which are nationally known and well regarded. -The sources Collins does cite as influencing HG's creation are half-hearted and any casual observation does not yield an "oh yeah, I can see that" response. -Numerous respected sources have found the similarities between the two works striking enough to warrant mentioning them -The two stories (when BR is observed in all forms) share a plot which is strikingly similar if not nearly identical (as noted previously) -Collins only reference to BR conveniently comes from her publisher AFTER she's supposedly written her piece, despite the fact that the publisher/editor had clearly heard of the BR before Collins turned in her draft. It seems bizarre that an editor would not check, upon hearing an idea that sounded to similar to a published work, to make sure well before a draft was turned in that the author was not merely aping or adapting an already published work.
I'm sorry, but her "That's my story and I'm sticking to it" mentality just doesn't go very far. It's not like BR was being written by another writer concurrently, where coincidence would be surprising but it's far harder to believe there is an element of plagiarism. No one would think less of Collins if she came forward and said "Yes I was influenced by Battle Royale" (and Running Man, etc.). In fact, I'd have more respect for her if she did that, because she'd be honest.
However, her current veil of denial simply turns me off to both Collins and the Hunger Games.
If you break it down to it's key elements and then some......... It is Battle Royale. People shouldn't be apologists for the body of work. it speaks for itself. The woman can craft and write all she wants, I won't knock that, but at the end of the day, it's still Battle Royale. Even Stephen King(who gave it a good review for the most part), called it out.
I'm not dismissing the similarities of the central concept of a death game originating before Battle Royale (though I'd argue with you including Enders Game and The Giver in there and will conceed to having never heard of Maze Runner), as I point out above... the broad strokes of her story follow Battle Royale pretty closely, with the exception that Collins lets her characters prepare for the game instead of being thrown into it. ( Government trying to oppress people, game of teenagers killing themselves, protagonist beats the game instead of winning, goes on to lead a resistance movement against the government, involved in a second game in a government attempt to stop them...)
My argument is very simple. An author has an idea. They do not sit down and write a novel without doing research, which often includes seeing if what is in their head has already been done. Regardless of whether she went so far as to google to central idea of her book, looking into games of kill or be killed, will invariably bring you across BR. It's not like it's an obscure book (which is a distinction I think only The Running Man shares when it comes to books with this central conceit) and it's entered into geek culture pretty pervasively, I'd say becoming more popular among contemporary Americans than any of the other works in the specific genre.
I'm sorry but it's easy to see several MAJOR differences between Battle Royale and previously published work like Running Man. But to really get at differences between BR and Hunger Games, you have to get into specifics, which, I firmly believe, indicates a familiarity with BR during the creation of Hunger Games.
I will not postulate on whether or not the author plagiarized Battle Royale. She claims she never read the book or saw the movie, and there is enough other material like it ("Ender's Game", "Running Man", etc.) that I feel uninclined to call her a liar. It is possible that the idea for this book originated without her ever having been exposed to Battle Royale. She lists both "1984" and "Lord of the Flies" as influences and I can definitely see that in her work.
With that being said if she did not plagiarize this from Battle Royale I feel really sorry for her, because this looks on the surface to be a complete retelling of the same plot with the only changes being the characters and the setting. As Brandon Hughes pointed out she hits all of the major plot points with only cosmetic changes made.