My best experience is having a game that opens up lots of goals or tasks, leaving you wondering which to work on next. This often satisfies whatever mood I may be in from day to day. Having a lot to do gives choices, and those choices should involve different activities.
I like to get in a group, everyone discusses what they need to work on, then we go out and see what we can accomplish. Dungeon crawls, camp an area for a rare spawn, source items for crafting to satisfy a quest, work on completing a map of a portion of the world, find hidden treasure, Explore, look for rare spawns….
Notice nothing I mentioned said go grind experience. All activities if done right, give experience and encourages to play, not grind.
So back to the question about best content for grouping? Having a challenging world that encourages group play is a must. Then having a LOT to work on that requires a multitude of different items to acquire, ties it all in… So to sum it up, would you rather be given a nice storage bag for reaching level 5 or would you rather it be given after a long, involved quest-line, that required you to explore portions of the world, earn rare drops, and put some work in to appreciate it when you get it.