Depends on the length of the fight as well as certain variables. While I would say that DRK would hold aggro the best for the majority, as others have said, WAR would have the best burst generation, so in a shorter fight it would be superior. Also, does the DRK have full MP and Grit and Darkside active before the fight begins, or do they have to activate them when the fight starts, which drains a hefty chunk of MP? For DRK, MP maintenance is particularly necessary to maintain aggro, which can lead to downtime on generation. Ultimately the result would probably amount to the WAR having the starting advantage, with the DRK pulling in a mid length run, followed by WAR and DRK switching between the two back and forth as WAR's burst capabilities come back and the DRK has to do MP maintenance, only for them to return to the previous after the DRK's MP is back up and the WAR's burst is over.....

I can only really speak in regards to these two as I have personally only played Tank a little, and most of that as a DRK. I've also seen very few PLD in post 50 content, so I can't speak greatly on their capabilities in terms of aggro control. It also doesn't help that the most recent PLD I saw was in a Garuda EX DF run, and he didn't even know what Shield Oath was, so I haven't had too many chances to run with particularly good ones.....

While we are talking purely on aggro generation here, I would like to point out that out of the three tank classes, I personally consider the WAR to be the weakest actual "tank" of the three. I've healed all three in higher level content, and I've never once had issues keeping PLDs and DRKs alive in big pulls or long fights, but WARs tend to have defenses akin to paper since most players prioritize pure STR builds to maximize their DPS with WAR. This is fine if you have a perfect group, but if you can't just AoE heal whatever damage the rest of the party takes, it can be a risk at times to stop healing a WAR to keep someone else alive. Just a side thought on the matter, and off topic I know......