WAR is fun because all of it's abilities can be used for something in a fight, not one ability can't be useful most of the time, holmgang being the exception. It also has a resource building and spending mechanic.
PLD is the least fun because it has a bunch of utilities that are very situational and so most of the time you'll be going through your combos and using basic cool downs as your utilities are only useful in specific situations and even then have long cool downs so although much more fun with the new abilities it still feels stale because it doesn't really have a mechanic.
Can't say much on DRK as i haven't been paying attention to it.