Probably the one with access to the Heal spell as a swift action. Can't really beat that once you've got it. Hell, spells are generally stronger than most other things in the game.
That said, Invulnerable Rager Barbarians with Improved Stalwart make rather impressive damage sponges.
And Crossblooded Arcane + Destined Bloodragers with Fate's Favored have rather significant defenses. The ability to cast any Bloodrager spell on yourself as a free action when you rage is rather immense. Specifically because one of the spells you get is Mirror Image, which can neuter physical encounters.
So I'd say that optimized Bloodragers make the better tank until late levels, where the Warpriest can Fervor-cast Heal.
Barbarians aren't really that "tanky" at all. They get hit. A lot typically. They persist by virtue of having high hit points from a big con score. The one build that can be tanky is by investing everything you can get into DR, but I wouldn't consider it to barbarians to be "generally tanky" despite having a build that can do so.
Warpriest can easily pick up heavy armor (without losing class features) and have all sorts of buffs spells that they can cast on themselves including healing.
In general the answer to questions is that classes with magic are generally better at stuff than classes without.
So I'd say Warpriest can be the most generally effective tank, while Bloodrager can be generally pretty effective too with good bloodline selection, and a specific barbarian build that's notable for their high DR.