For years, AbbVie's (NYSE: ABBV) powerhouse product Humira has held the top-dog position of best-selling drug in the world. In 2016, this anti-inflammatory drug brought in over $16 billion in global sales and is expected to remain the world's top-selling drug out to 2020. However, being lead husky means the competition is always nipping at your heels, and competitors are itching to knock Humira off its perch.