I disagree that teachers shouldn't have favourite students. It's pretty easy for me to determine early in the school year, which students get the best of my attention and which ones will benefit most in the long run from a little favouritism.
My favourite students are the ones who are the most attentive and involved in the class. I play to this section of the crowd. If the rest are quiet, I leave 'em alone.
I'll never forget a TV interview that the head coach of the Dallas Cowboys gave (Jimmy Johnston) after he had fired a player for sleeping in a player's meeting. He was asked if he would have fired Troy Aikman (the quarterback) if HE had slept during the meeting.
His answer was: "Of course not. I play favourites."
Made sense to me.