before 2002 none I was unable to locate any available superhero mmo.
2002-2004 Dark Age of Camelot It filled the time before City of Heroes released. The RvR fights (their version of PvP) was massive and open-world.
2004-2012 City of Heroes Great experiences with a variety of characters, teaming-up on missions, and overall positive community (both ingame and forums).
2012-2015 Star Wars: TOR I loved their companion system, but wished for being able to choose your own companion for regardless what class that you played.
2015-2017 Marvel Heroes So many options on how to customize your chosen character's gameplay and build. I was disappointed that it was shutdown.
2017-present Guild Wars 2 Very casual. I enjoyed the open-world and cooperative aspects of it.