Whichever one boosts the most important aspect of your toon at any given time. With all available bonuses:
- Vigor of Battle: +20 heal amp +100 HPs (boosted by Tenacious Defense + Strength of Vitality)
- Unbreakable: +5 AC +20 PRR DR 10/epic; w/Fanaticism your weapons gain True Law (+1d6 dmg vs non-lawful targets).
- Stand Against the Tide: +2 to-hit, knockdown immunity, and "+1 melee damage, 20% threat generation, 5% fort, +1 reflex/fort/will saves. This bonus stacks up to four extra times while standing still." With Anoint Weapon you gain Holy (+2d6 dmg vs evil) which also counts as good-aligned for DR purposes IIRC.
I haven't epic-tanked in a long time, but normally I would use SAtT (with either Precision or Power Atk) for the extra DPS & threat with occasional shifts into Unbreakable for extra PRR when tanking (particularly if I twisted Improved Combat Expertise from LD and switch on CE). VoB is the weakest of the Sentinel stances, IMO, but extra HPs & heal amp is never a bad thing.