NFL 2K was a franchise that was amazing in its own rite and pushed Madden to mainstream success (more on that later). In addition to game innovations, the franchise made playing fake football a hell of a lot less expensive. In 2004, then ESPN NFL 2K5, dropped prices for both games, Madden was $29.99, 2K5 was $19.99. That's both games for less than a new Madden today.
But you are concussed if you don't think 2K had its fair share of crappy ideas: First person football where you have no feel for what's going on around you, the Crib where you could lounge around watching Sportscenter ads, and playing Steve-O which was....we don't even know.