Yoshi-P has already stated that classes are more for light parties, and that the upcoming jobs are designed for full parties.

Unfortunately all we have is classes atm, gla mrd and pgl are all about the same tanking wise, some being better at tanking some monsters than others, but not a large enough difference for any of the 3 to clearly outshine the others.

Gladiator has a shield for blocking, and can evade and parry with a lower chance than the respective classes.

Marauder has a huge HP pool and parry, but can also evade about the same as gladiator.

Pugilist has evasion, and partial parry, which while this may make them the weakest defender, they also have the most devastating returns for attacks they evade.

Now some say, well gladiator can wear all the best evasion gear and use feather foot to further enhance this. They can also get a high parry weapon and use foresight to enhance parry as well. Basically it comes down to them being the only war class that can wield a shield, but at the same time, gladiators usually sacrifice lots of their damage and weapon skills for more defense and enmity abilities. this makes them the slowest to grab a good amount of hate. They also hold hate poorly because steady damage holds better hate and for longer than these enmity abilities. Even with max HP gladiators can still quickly go down on Batraal if swords come up at a bad time, or any number of things.

Once jobs are introduced we may see a clear leader but for now each of the 3 tank classes has it's strength and weakness.