Guardian: Best single target threat. Worst aoe threat. Worst dps. Mid range for passive mitigation, cooldowns, and utility. Best snap agro. Worst mobility.
Shadow Tank: Best utility. Worst passive mitigation. Mid range on both threat types and aoe dps. Best single target dps.
Vanguard: Best aoe threat. Best mobility. Best passive mitigation. Worst cooldowns and utility. Worst snap agro.