I don't have the knowledge of this game to be able to get into the math of it and I don't have access to all of the seemingly undocumented mechanics of various skills and systems in the game. So clearly the observations that I will be making in this post will not be backed up by math, quantum physics, the Scoville Scale or powerpoint slides.
But I do see a huge disparity of tankiness from hero to hero and I am very surprised as to who stands on top when I build them with various levels of mitigation.
On one extreme you have Deadpool. His kit is pretty awesome if you build him right with a 25% bonus to defense and dodge and in theory 20% to deflect if you spam flesh wound. I use a slightly tankier version of Eccoman's CDR build and I'm pretty much invulnerable. I have to be a little careful when I do some higher level CDR's but this is definitely my "Watch Netflix on the other screen while I mindlessly farm" hero.
Then I have Kitty Pride, who yes, I get it, she is a completely different playstyle taking advantage of stealth and well timed invulnerability but statistically she shouldn't get smoked when I play her when I just face tank for a few seconds. She has about the same benefits that Deadpool has from passives. His numbers are a little higher for defense and dodge and has that heal which I never use but she also has that 8% DR in Intangible. But I build her a lot tankier in general than I do DP. She has about 10% more damage reduction the way that I build her, her effective health is way higher and it's not that I get completely smoked but the difference is god mode versus "I better stick and move and pay attention or I will get smacked down" and she does less damage.
Colossus is another one who surprises me. I shelved him for a long time and decided to try various builds. I tried some silly build someone posted that almost completely focused on making him as tanky as possible and of course he did no damage. But I was immortal. I dropped it down to where my effective HP is 200K and DR is in the low 70's and while he can still take a pretty good beating, even with those ridiculous numbers, I have 4-5 other melees who can do twice his damage and tank as well or better. In CMM I have to use med kits with 200K and 70% dr.
By no means should this game be played as an invulnerable tank especially if it means putting up a 20 minute TTK and not being able to contribute to anything (I'm seeing this in cosmic patrols lately). I prefer to play my Elektra almost glass cannon who seems to be able to withstand a massive beating or just simply avoid damage because she is always jumping around.
Again, the numbers are beyond me. I'd love to hear the thoughts of others on this who can school an old man eager to learn more.