Of all of the fighter class can tank roughly equal. They all have difference styles though. Mostly their "flavor" abilities.
Each class will be main tank just fine, alot of their specific abilities effect when they are not main tanking.
Berserkers, for example, can go berserk for extra attack, but weaker defense. Without berserk, they tank just as well as a guardian does. But when not main tank they can go berserk and do extra damage. Currently the hate of alot of their abilities is super intense, which makes it hard for them to go berserk without stealing hate.
Guardians are kinda the opposite. Most of their flavor abilities are based around shielding allies from attacks. And if they were tanking, that shouldn't even come into play. Sure there are adds at times, but where the guardians powers really shine is backing up the main tank by boosting their defenses.
As stated many times, each and every fighter can tank. Some people will suck at it, especially brawlers. Brawlers can tank just fine, but beware of those L65 Eq1 monks who are all about DPS and will refuse to tank, or suck hard if they do.
Alot of the fighter flavor abilities were added so that there was a real point in having two or more fighters in a group. Only one will be main tank, and the flavor abilities kick into play for the secondary tank. In this case, it's extra damage for the berserker, or the abilities to buff the main tanks defense for the guardian.