Often, fantasy owners have a tough time picking someone for their flex spot. Following the running back and wide receiver rankings are straightforward, but when it comes to flex it gets tougher. The rule of thumb is that in PPR formats, use a wide receiver in the flex spot whenever possible. In non-PPR formats, use a running back. This is simply a guideline but can result in the most points from the position.
Also, know your league settings. These are flex rankings for running back, wide receiver and tight end only. If your league has a "superflex" or a flex where you can play a quarterback, often that would be the best option. Treat this kind of format like a two-quarterback league.
— Rankings by Sarah Lewis, who is part of the Athlon Contributor Network and lives, eats, and breathes fantasy football. Have a fantasy football question? Send it to her on Twitter @Sarah_Lewis32.