I disagree, Fighter's are better Defenders. I'm not saying that fighter is a better class, but if all you're measuring is how good a defender each class is the fighter is the clear winner. Though it may be that we're arguing from different definitions of Defender. For me being 'sticky' is the defining element of the defeder role.
I tend to evaluate the game from an action economy impact perspective. Leaders give actions to allys or make allies actions more effective, while defenders and controllers take actions away from enemies or make enemies actions less effective. Strikers are an interesting case in that their action economy effect is backloaded, but they're not really material for this conversation.
Assuming the above definition is valid, the question of who's the better defender comes down to who causes their enemies to use their actions less efficeintly. An attack against a defender is less efficient than an attack against another party member, defenders incentivise enemies to attack by marking them, therefore the defender that can incentivise more enemies to attack him, by marking more enemies, has an edge in this catagory. Point Fighter.
Wow. I think you hit the nail on the head. Sticky is not the definition of the defenders role in my opinion.
The defenders role is to control the damage flow of the enemies to the party, minimizing the total damage the party takes and spreading the damage they do take in the least harmful way possible (as a side note the controllers role is to minimize the effectiveness of enemies actions by forcing them into sub optimal strategies.)
Stickiness is a means to that end, but not the only one. If you're definition of defender is a ball of glue and duct tape then fighters are the idea defender. But there are plenty of ways to do the same job.
Cadfan has covered a lot of the responses I would have given. Here's a few more.
Damage = one way to be sticky
If the enemy shifts away the fighter gets an immediate interrupt attack, and unless another of the fighters ally was within shifting distance the enemy still has to move after his shift, leaving him no action left to make the attack that would trigger the palladin's punishment. if the enemy moves away the fighter gets an OA, which might prevent the movement altogether, which means the enemy won't be attacking an ally, so no pally damage in that case either.
Want to get away from a fighter? move, eat the attack and get your movement action ended, then charge away. No more OA's available to the fighter to stop you and you've moved away before the attack so the fighters combat challenge can't do anything. Not quite as sticky as advertised.
In order to get the paladin's defender method you have to look at the healing surge situation. Lay on Hands takes Paladin hp and turns it into other party member hp. So the damage that your allies take actually gets taken by the Paladin, allowing him to retroactively act as a defender. Here's where that extra healing surge comes in handy. (I know the "healing" fluff of it makes it seem leadery, but don't be fooled. The mechanic is defender through and through)
Better than the fighter's OAs? The fighter likely has also prioritized Strength, making him equally likely to hit, plus the fighter's OAs have an additional action economy impact that makes them a better defender.
I meant better than a Cha paladin. That was unclear. My bad. Fighters get wisdom to their opportunity attacks plus their +1 to hit from weapon type. There's no way a paladin could outstrip a fighter in OA's.
I'm curious what you think about all the abilities Paladins have that allow them to literally force an enemy to attack them instead of their ally, or to take half of the damage for one ally and other similar effects. Can't get more sticky than that. The enemy is litterally stuck.
Fighters defend by focusing on the enemy.
Swordmages defend at the point of contact.
Paladins defend by focusing on their allies.
Different methods, same role.
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