Protection
When compared to attacking and thwarting, Defense in this game is more understated and considerably less flashy. By extension, Protection is also the least flashy of the 4 aspects because it does not add or remove tokens from the board like the others do. But by having a strong defense, it gives you a solid foundation in which to leverage a win with your other cards.
The defend action has a reputation for being the weakest of the basic actions, and it is deserved, as it is often worse than recover but Protection turns that action into a Strength. Protection has the most ready effects of all aspects, allowing you to mitigate damage, ready, and still control the board.
But Protection is not only about the defend action. Protection has a collection of cards that mitigate damage outside of the defend action, stun, and also has a strong sub-theme of healing. It can make a deck that does not defend at all.
Protection is all about mitigating damage and the main advantage of doing so is prolonging the time that you are in hero mode. This has the benefit of reducing the amount of times that the villain schemes, but also some heroes simply function better in hero mode and benefit from the added defense. But Protection lacks in active threat removal, so it can struggle against side schemes and also has limited damage capabilities (tho it is not the worst). Since pretty much all decks share these strengths and weaknesses, they are not mentioned with the individual decks.
Decks
There are 2 main decks with a third hybrid deck, which is essentially a combination of the 2. The names are mine as there are no "official" ones.
Defense Protection - With the addition of Dr Strange's Pack, Defense Protection received a big boost. Defense Protection runs both Indomitable and Desperate Defense to ready as much as possible after defending. DD, along with Energy Barrier, Armored Vest, and your basic defense provide the mitigation. If using Black Widow, she uses preparations instead of indomitable in order to ready with Synth Suit.
It is most effective with heroes who can achieve 4 defense with Armored Vest, as they can more easily trigger Unflappable, but Heroes with 2 base defense can use this build also. 2 Def Heroes are more reliant on getting Armored vest out as well as energy barrier in order to fully prevent damage with Desperate Defense.
The cards you choose outside of the core defense package give plenty options for variants. You could run med teams along with a few key allies like Nova, Luke Cage, and Heimdall. You could run Iron Fist, Tackle, and Clea and essentially be a tank for the whole table, and you can run utility cards like Get Behind Me. There are a few options here.
Strengths: Efficient defending, ready effects, can mitigate multiple attacks in a villain phase
Weakness: no real strengths except defending
Best with: Black Widow, Captain America, Spider-man
Sample deck: pure - https://marvelcdb.com/deck/view/34695
Tank - https://marvelcdb.com/deck/view/34465
Hybrid Protection - This deck devotes less cards to pure defense, running either Desperate Defense or Indomitable, rather than both in favor of more stuns and offense.
Strengths: A more balanced deck
Weakness: Still doesn't solve Protection's weaknesses
Best with: Black Panther, Captain America, Hulk, Thor
Sample Deck: https://marvelcdb.com/deck/view/34246
Sustain Protection - This is a protection deck that aims to mitigate damage and stay in hero mode without using the defend action. This deck uses cards that reflect damage, heal, stun, and has greater emphasis on allies. A variant exists that leans fully into Protection's excellent suite of allies combined with med team for strong ally healing.
Strengths: Mitigation without having to defend.
Weaknesses: More expensive for similar results compared to Defense Protection.
Best with: Iron Man
Sample deck: https://marvelcdb.com/deck/view/34134
Cards
Legend:
S - Very strong card that should be used in every Protection deck.
A - It is not used in every deck but it is a defining card in a specific deck.
B- A good card used in many decks but is not essential.
C- An overall weaker card but has synergy with specific heroes.
D- An overall weaker card that does not have strong synergies but is still marginally playable.
F- Not worth the paper it was printed on.
Please take the ratings on cards from un/newly released packs with a grain of salt as they are based on speculation and limited game-play.
Black Widow - B - Black Widow is a toolbox card, nice to have, but she is not essential.
Brother Voodoo - C - Expensive for the stats, but the ability is good with certain heroes, like Black Panther.
Clea - A - Stats aren't great but a reoccurring blocker is really good.
Iron Fist - S - 6 dmg, 2 stuns, and a block is just insane value. There are a couple heavy defense decks that don't need stuns, but other than that, he is just too good.
Luke Cage - A - 2 blocks, and 8 damage is really good, really good for augmenting your damage, just needs a lot of turns for full value.
Nova - A - 1 resource for 2 damage is efficient and is strong for dealing with minions.
Counter Punch - B - very efficient damage for heroes with 3 atk.
Desperate Defense - A - +2 def and a ready is extremely efficient.
Expert Defense - D - Made obsolete by Desperate Defense, and even before, would often be overkill.
Get Behind Me - B - Like Black Widow, it is a toolbox card that is nice to have (especially if you are prepared for it), but not essential.
Momentum Shift - B - Protection doesn't have a tonne of ways to deal with minions. This is good for that, with healing on top.
Preemptive Strike - B - Solid damage mitigation and damage in one, particularly for Sustain decks.
Second Wind - C - Not the most efficient card, but decent with higher hp heroes who want to stay in hero mode.
Tackle - A - Stuns are really strong (particularly at the lower player counts) making this a very good card.
The Power of Protection - S - Some Protection decks run a lot of cheap cards but most will have enough 2+ cost cards that this will be worth it.
Med Team - A - This is the most efficient heal card in game and can heal both heroes and allies. Really strong.
The Night Nurse - B - This is a great toolbox card and it is worth having in most decks but it is also possible to play this and then never get a status effect.
Armored Vest - A - For decks that use the defend action, this not only provides great value over the course of the game but will also make it easier to trigger Unflappable and Desperate Defense.
Defensive Stance - C - 2 cost for mitigating 3 damage is not very efficient. It does not require defending, but the Sustain deck doesn't really need this card (and would frankly be better with Second Wind). This is really only good with Black Widow who can use this to trigger Synth Suit, Unflappable, and her ability.
Electrostatic Armor - A - For Defense decks, this will provide great value over the course of the game. It is a 1 cost tech for Iron Man, although he doesn't typically defend.
Energy Barrier - S - This is not a crazy powerful card but it performs an important role in all of the decks (general mitigation for sustain, tech for Iron Man, and helps prevent all damage in defense decks), so this one is not to be skipped.
Indomitable - A - While overshadowed somewhat by Desperate Defense, this is still good as you can run both and this card is also easier to use along side stuns and ally blocks than DD.
Unflappable - A - A key part of Defense Protection and provides a pretty big pay off for fully defending.
Heroes
Black Panther - Wakanda Forever is efficient but doesn't do a tonne of damage, so he can struggle for damage with this build, but the retaliate will do a lot of work and his resource generation will be help pay for Protection's expensive allies.
Black Widow - Synth Suit combined with the mitigation tools that Defense Protection runs turns her into a nigh-invincible beast who can ready nearly every turn.
Captain America - 3 defense, retaliate, and stuns in his signature kit combined with protection gives him an unrivaled amount of mitigation potential. In solo, he can stun the villain for turns on end and in multiplayer, he can protect the whole party with defenses and stuns.
Captain Marvel - While she can potentially get 3 def with her helmet, a Sustain deck will be more consistent and the deck does have a lot of energy resources. The biggest disadvantage with this build however, is that her excellent Alter-Ego ability goes to waste.
Dr Strange - Dr Strange needs extra defense to truly leverage his excellent invocation deck and signature kit. Protection can provide that defense however Leadership can also provide the defense he needs and offer greater synergy.
Hulk - He absolutely wants to stay in Hero mode as long as possible and Protection can help him stay there however, it can be difficult to afford both his Signatures and the Protection cards. But if can get some basic resource generation out to enable that, he will shine. He also requires a lot of physical resources and Protection's non-unique cards that have physical resources are Counter-Punch, Tackle, and Momentum Shift. With these cards, he leans more towards an offensive oriented hybrid deck.
Iron Man - The Sustain deck has several factors that fit Iron-Man perfectly. Energy Barrier is a tech so it can allow him to get to max hand size faster, the deck has a lot of mitigation and healing to allow him to stay in hero mode once he flips, and it has a lot of energy resources for Repulsor Blast. A very solid deck.
Ms Marvel - Her ability combined with embiggen can make good use out of Protection's attack events like Momentum shift and Tackle and Shrink + Sneak by gives her considerably better thwarting than the average Protection deck. But her excellent alter-ego ability, Personas, and her Recover 5 are under utilized in a Protection deck.
She Hulk - She has stun lock potential, her hp is an advantage, and her 3 atk synergizes with Counter punch, but she wants to flip every round which is anti-synergistic with Protection's strength of prolonging the amount of time spent in hero mode.
Spider-Man - 3 def and his hero ability makes him well suited for a defense protection deck. Like Captain America, he offers a very high level of mitigation however, his thwarting capability is pretty low making this better in multiplayer.
Thor - He loves being in hero mode as long as possible so Protection is a good fit. He also gives Protection great offense and minion control that it normally lacks. Thwarting is lacking however, making this a challenge in solo.
Overall, Protection can be effective but it does tend to be more restrictive as for which heroes are good with it as well as which cards you need to run. It is a one-dimensional aspect, but it does that one thing really well and with the right Heroes and Group Comps, it will do work, even if it is the supportive, behind the scenes type of work.