One thing to keep in mind (reading the above comments) is what level PLD gets abilities. For example, Clemency doesn't come until 58 (and Raw Intuition until 56). Before 58, PLD has zero healing. It doesn't have healing in its rotation until level 84 (EIGHTY-FOUR, no that was not a typo) when Holy Spirit/Circle start healing you.

Up until level 58, PLD has zero healing (somewhat counterintuitively). WAR and DRK both have healing from the -3 of their 1-2-3 combos as soon as they get those (level 26) and GNB gets half that healing but paired with an equal amount of shield from their -2 from level 4 on. Though you can't start with GNB until level 60 anyway, so...

This is pertinent if you're doing stuff like Palace of the Dead, as PLD doesn't get any healing until you get near the level cap (60) of that content.

.

Once PLD gets Clemency, it's a beast, though. I soloed some HW Extremes back in ShB and some people were soloing some SB stuff. But they were also doing that on WAR.

WAR and PLD are both good choices. Right now, WAR has basically PLD's sustain + more damage to boot, so it's pretty far and away the strongest Tank for any content, including soloing. PLD is pretty solid for soloing and for Tanking in general, and probably has slightly higher defensives than WAR. So it kind of depends on which you prefer. DRK you can't unlock until at least level 50, but yeah, it's great in level 90 content and Ultimates due to the way its damage profile and burst work, but it's less good for soloing stuff and can be a bit of a pain to level until you get your defensives in the 70s. GNB is actually a pretty good "all around" Tank, all things considered. It has less sustain than PLD, but gets its healing earlier, and it may do the best damage at the moment, or close enough to that it doesn't matter (though Tank damage isn't that wildly apart for the most part anyway...)

.

They're all fine for whatever content you want to do, but DRK will have a harder time with soloing and stuff like that. WAR probably is the easiest in the game right now and has good damage and defensives/healing as well. PLD and GNB are what happens if you take that and go more towards defense and healing (PLD) or more towards offense (GNB). PLD's also easier than GNB; GNB is probably the hardest Tank to play if you aren't great at burst and high APM (DRK second hardest).