I love the Intellivision. The biggest downside - for me - is that most of the early-midlife games are 2 player. I could fire up my ColecoVision or Atari and play when I was by myself but Mattel insisted on too many multiplayer games early on.
Some of my favorites to play: Advanced Dungeons & Dragons©: Treasure of Tarmin, Advanced Dungeons & Dragons©: Cloudy Mountain, Microsurgeon, Space Battle or Space Spartans.