The most watched episode of a TV series in the 2010s was . . . the premiere episode of “Undercover Boss” on CBS in February of 2010.  It had 38.7 million viewers.

Super Bowls don’t count because they aren’t a TV series.  However, the reason this episode garnered so many viewers is that it aired immediately after Super Bowl 44.

This is the Super Bowl where hell froze over; the New Orleans Saints won the Super Bowl.