All the forums seem to have their specific niches. Jetcareers seems best for the up and coming pilots and is a great service for people just starting out in the industry. Flight Info used to be okay, but after the original owner tragically died in a plane crash it went downhill very fast and is now populated by too many trolls. Most of the good members left to go to ProPilotWorld where no trolls are allowed because all members are checked against the FAA database and must have a commercial certificate or higher. Moderation is done only when necessary and no personal attacks are allowed. It's a very good resource for corporate pilots and a great way to network in that segment of aviation. It also has good airline specific discusions but not to the extent of Airline Pilot Central. PPrune is good for the international arena and easily has the best technical discussions, but far too many personal attacks for my taste.
If I were go by my personal post count in the last two years then my vote goes:
ProPilotWorld
Jetcareers
Airline Pilot Central
PPrune
Flight Info
The disclaimer being that I'm a moderator on PPW
Typhoonpilot