If fun is my mission, then my preference is a group ride, and with leader speeds slightly above mine, where constantly chasing a wheel and trying to not get dropped is "fun". Getting dropped by people you know and ride with weekly is incredible motivation to eat, drink, and train as best as I can so my weight and fitness improves incrementally each week, thereby reducing the frequency that I get dropped. In these times of "forced" solo riding, that motivation just isn't there for me, and my discipline and fitness has suffered as a result.
If a calm/meditative/recovery tempo ride, or the exact opposite which is a fitness improving HIIT, is the mission, then solo is usually where it's at, unless you find a small group that does HIIT or Strava segment chasing.
Group rides are also good for sharing bike repair resources and mech. experience, for oddball things that come up.