In CO, defensive passives are among the biggest things that make a tank a tank. Picking the right defensive passive for your playstyle is important. So without further ado...
DEFIANCE: Used by The Behemoth AT, Defiance is simultaneously one of the best and worst defensive passives in the game. It's a stacking buff (like an offensive toggle), that provides ~15% resistance to all damage per stack, scaling with constitution, stacking up to 6 times for a max of 90%. It adds stacks whenever you take hits.
90% is more resistance than any other defensive passive puts out, which is good. It also scales with Constitution, which is the primary superstat for most tank ATs. Problem is, it stacks only when you take hits. So for that first hit in a fight, you only have 15% defense for, and you'll need to be hit five more times to reach maximum defense. Ranking up Defiance makes it easier, by starting you with 3 stacks instead of just 1, but that still means you have to get hit a minimum of three times before you're fully protected against further damage.
This means you are
extremely squishy in the first few moments of combat. You need to be in large groups of enemies to gather defiance stacks quickly, but that means getting chipped away at before your defenses are up fully. You begin the fight at a disadvantage compared to other passive defenses. Further, since Defiance has no flat damage mitigation, you can be chipped to death by large packs of weak enemies, which is painful and annoying. Against packs of henchmen and villains, you'll be taking 20s and 30s where an Invulnerability tank will be enjoying 1s and 2s. You need to wipe out small enemies quickly to avoid getting chipped to the point where the big baddies can outright splat you with their massive damage attacks. In addition, you are more vulnerable to maintains and damage over time effects; an Invulnerability tank gets their flat damage reduction on each hit - you only get the percentile reduction.
Defiance can be good if supported with lots of self-healing ability, like Resurgence, Conviction, and Palliate. On its own, its habit of being squishy when first entering combat can easily get you killed.
INVULNERABILITY: An excellent passive defense, useful for almost any character. It provides you with 27/33/40% resistance, plus scaling damage reduction based on your Super Stats. Since ANY super stat can contribute to Invulnerability, which superstats you have selected don't matter. You're not strongly advised to take CON as a primary, as with Defiance or Regeneration.
The advantage of invulnerability is that the flat damage reduction can potentially reduce weak attacks down to a measly couple points of damage. An enemy that would do 50 damage can very easily be reduced to piddly single-digits. Further, the flat reduction is applied after your resistance, so it can kill a greater part of the reduced attack. A nice side effect is that DoTs and maintained damage powers are particularly weak against you - you get your flat damage reduction on each tick that they deal damage.
Invulnerability is not without its weaknesses though. It does not provide as much resistance as Defiance is capable of, and that flat damage reduction doesn't mean a lot in the face of enemies capable of dealing large, highly-damaging single-hit attacks. Boss enemies (Super Villain and higher), tend to take a lot out of you - it's a good idea to kill them first, since their henchies and other underlings really can't do much to bother you.
REGENERATION: Somewhat of an odd duck among defensive passives, Regeneration doesn't actually reduce incoming damage at all. Instead, it provides a perpetual heal-over-time effect, which scales with your super stats.
Regeneration is vulnerable to alpha strikes. You do not want to take large amounts of damage at once, since you need time to recover from each hit that lands on your squishy-yet-fast-healing-body. You need to pace the amount of damage incoming at any one time. Your regeneration gets more intense as you take damage, so like Defiance, you are squishiest when you first enter a fight and get better as the fight goes along.
Blocking is key to making Regeneration useful, since a good strong block can potentially reduce enemy damage to the point where you can heal faster than they can hurt you, allowing you to essentially restore your HP mid-combat by blocking. Just be careful, as blocking rapidly 'resets' the intensity of your regeneration, meaning you'll need to take a few unblocked hits to get it going again. If you take Regeneration, take
Resurgence with it. Not only is Resurgence a good instant-heal, but it also ramps up your regeneration intensity when you use it.
PERSONAL FORCE FIELD: Personal Force field provides you with a permanent Damage Shield, that acts as a second layer of protection on top your HP - a sort of buffer of "overhealing" that needs to be blasted off before you can actually be harmed. This shield is constantly restored over time, and blocking causes it to be restored
even faster. Both the shielding amount and the rate it regenerates is scaled with your Super Stats.
Its downside is that its regeneration rate is slowed by taking damage. Thus, it works the opposite of regeneration - taking unblocked hits makes you squishier, while blocking toughens you up. This actually is advantageous for you, since you can turtle up under your Block to let your shield repair itself. Unlike Defiance and regeneration, you are toughest at the beginning of combat.
Neat thing about Personal Force Field is that picking up yellow and blue boosts will restore your Personal Force field, in addition to applying their normal effect. I highly recommend, that if you take this power, you also take
Field Surge as well.
LIGHTNING REFLEXES: Lightning Reflexes increases your dodge chance and avoidance - making you more likely to dodge hits, and reducing the damage you take when you dodge a blow.
On its own, Lightning Reflexes is pretty meh. With dodge-centric gear to increase your dodge rate, and other powers that provide dodge and avoidance, such as
Evasive Manuevers,
Lead Tempest,
Thundering Kicks, and
Parry, Lightning Reflexes can trivialize a lot of incoming damage.
I highly recommend you take
Bountiful Chi Resurgence with its advantage, as well as
Form of the Master with its advantage. Both of these combined will allow you to turn your rampant dodging into extra offensive strength and healing, making you even more formidable and durable.
