While I'm a huge fan of KF, I found Mass Effect 3 (yes, Mass Effect 3) multiplayer to be more fun and well produced, which was perhaps the greatest surprise for my gaming experience in past few years.
It's only up to 4 players, it doesn't have dedicated server or custom map support, but it had equally satisfying gunplay and tons of characters and weapons, each has their own unique tactics and customization to apply, which were just great.
I recently tried Warframe but it was no where near as satisfying as ME3MP.