The game that came with the system - Super Mario. Took me about a week or so to beat the game the first time and I played it so often that I started going for fastest time to beat the game..then highest score possible..and then lowest score possible.
My wife's favorite game was Solomon's Key...I was at a garage sale and found one for her...brought it home and it wrecked my NES system..hasn't worked since I plugged that game in.