Light novel: quite good and a lot more focused. A good portion of the stuff that happens in the manga/anime was never in the original novel. However there's a few good scenes from the novel not in the anime/manga. Recommended.
Manga: Great until the last few volumes when it dissolves into a complete mess. Characters start acting downright schizophrenic and the ending is unsatisfying. Still recommended though.
Anime: A mix between the novel and the manga, with the novel's ending but a number of events from the manga. Slightly toned down compared to the manga. Recommended.
TLDR: All of them. You'll get a different experience each time.