Most intuitive favorite icon
Which conveys the concept of favorites better? I'm working on a news reader, and this could be a desirable feature.
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The best choice here depends on when a user would use the 'favorite' function.
If the goal is simply to flag an article so it can be found later easily, like marking a page in a book, the bookmark metaphor is a good one. Most web browsers use the bookmark term/icon because the function is to make a page easy to come back to.
On the other hand, if 'favorite' is more broad, consider using the star (or a heart like Instapaper uses). In my newsreader, I use favorites for both bookmarking articles I want to find later and for marking articles that I simply really like. At that point, the bookmark metaphor no longer makes sense: you don't mark every page you like in a book. Additionally, some newsreaders use favorites as a social feature (e.g. tweeting when you favorite an article or making your favorites list visible on a profile page). At that point the bookmark really turns into a forced metaphor, and, in my opinion, should be removed. A star or heart, though, is broad enough to work well here.