I don’t want to rehash what typically turn into heated debates on the best tank, but here are some major considerations:
Plate armor will not improve your touch armor class. That was very important in PM KM’s end. I don’t know whether it will be important here. If it is, plate will always underperform a dexterity tank because plate AC and shield AC are ignored.
There is some amount of AC above which more AC is worthless. I don’t know what that number will be in WOTR, Chapter 5,
But there will almost certainly be many plausible builds that can get there. Once you reach that AC, your real risk of dying is a natural 20 roll. There’s a 5% chance of that occurring every attack. To reduce that 5%, you should have Blur/Displacement and Mirror Image, which will take it to something like 1.25%. Because Blue/Displacement/Mirror Image are so impactful your residual risk, classes with access to mirror image are “better”.
Tanks that stack dexterity over strength are “better” because that stat converts to AC and also improves touch AC. Plate armor limits your ability to stack dexterity and, at least in PF KM scaled much slower than dexterity growth.
The “best” tanks will splash certain monk or magus archetypes to gain feats that convert other stats, ie wisdom, charisma or intelligence to AC as well. Even without items, there are LVL 2 buff spells that make lengthy, +4 buffs available early.
Without items, no armor dex tanks will outperform because mage armor and shield give a huge edge (+8 AC on top of dex bonus). Note, you’ll need an alchemist to give the tank shield.
The crane style monk feats are extremely powerful. Even without a robe, those feats and the wisdom/charisma to AC feats make a 1 lvl dip in monk worthwhile. You lose no BAB and get a free feat, while gaining access to excellent feats and a second stat for growing AC.
In any event, a good tank will include some combination of the following classes:
1/2 monk (either charisma or wisdom to AC class, get crane style feats)
2 paladin (charisma to saves)
4 levels of thug (dazzling, dex to damage)
XXX Vivisectionist (stop on a level where to get a mutagen upgrade, gives access to shield to others)
XXXX Sword Saint (allows conversion of int to AC, will help meet requirements for duelist)
8 or so Aldori Defender (used with a duelist prestige and to mitigate fighting defensive penalties)
4 levels of dragon disciple (for a strength build)
I also like a pure dex eldritich scion with 1 lvl monk.