Now that I'm measuring everything by value added, I should give a simple guide that underlies my thinking on the matter.
If there's a book, manga, and anime version of the same story, which should be consumed and which best ignored? This order to my halls of fame answers this question and all others of the same sort:
Anime > video games > visual novels > (non-anime) movies/tv shows > manga > books.
This is why J.K. Rowling's Harry Potter series is ranked so lowly in my books section, it's trumped by the movies so it has very little value added left to boast about.
Likewise, Frank Herbert's Dune series is trumped by the video games, movies and tv shows that already soak up most of the value of the Dune franchise before you even get around to the books.
If an anime were ever made of Utawarerumono 3, the video game would lose its value and need to be replaced by something else.
Even Herodotus suffers from a lack of value added, because his history covers the same period of time as the magnificent 300 movies, which trump books.
Luo Guanzhong, author of Romance of the Three Kingdoms, has very little value added left due to the Dynasty Warriors video game franchise stealing most of his thunder.
Of course, there are exceptions. When an anime does a terrible job of adapting its source, like with Freezing, the anime isn't as valuable as the manga. Ideally all butchered anime shouldn't even be in my rankings, but that's not currently the case. When there's a very low ranking anime in my rankings and that same series is very high ranking among my other halls of fame, that's a time when the anime version is less valuable than the original.
But generally speaking, people should start by watching anime, then playing video games, then visual novels, then watching movies and tv shows, then reading manga, and lastly reading books.
There's also the strategy of first watching the anime of something you like -- let's say Clannad. Then seeking out any additional Clannad products you can -- in which case you could read the visual novels. Or you could watch Rokka no Yuusha and then switch over to the light novels for the rest of the story.
You can carve up my hall of fame into franchises and do it in franchise order, instead of medium order, and that makes perfect sense too.
The one thing you shouldn't do is waste your time with an inferior version of a product when there's already an available superior version floating around. Life isn't long enough to waste your time with subpar repeats of already existent things. In Magic the Gathering cards like these are described as 'strictly inferior' and should never see use in any deck at any time.