Disclaimer: This Guide will always be in progress & based on my learnings & understanding. I might be wrong on few points which you can correct & give suggestions. Based on Servel Level 160 & R5
My comprehensive Mods & Mods Upgrades Guides for heroes can be found here. All suggestions are welcome.
Important Stats

Evasion: Evasion gives heroes a chance to avoid enemy disables. Each point of Evasion above the enemy’s skill level increases your chances of avoiding a disable by 1%, up to 90%. If a disable is avoided, any other effects on the skill still resolve.
I will explain this by taking an example, now let’s see Evasion of Cat at R5 level 160 with a max badge is 228, now you have seen that Minnie’s charm or Kristoff freeze doesn’t work on Cat, the reason is high Evasion. So, as per the definition in order to charm the Cat, you need to level up Minnie’s Green Skill.
As per definition 1 skill level above hero level will increase your chance of disabling the cat is 1%.
So, 228 ( evasion )  160 ( skill level ) = 68 level. So, in order to charm the cat by 90 %, you need to bring that skill level to 228, which means +68 level ( can be done by using mod upgrades ) 
Tenacity: Tenacity reduces the length of stuns and disables a hero receives. Each point of Tenacity above the enemy’s skill level reduces the length of stun or disable by 1% up to a max of 90% reduction.
So, it works the same way as evasion, you need to increase your skill level to minimize the tenacity stat, lower the tenacity stat & your skill will work more time.
Max has high Tenacity so in order to freeze Maximus for more time you need Kristoff’s blue skill to as near as his Tenacity Sata 
Skill Power: Skill Power increases the damage that some, but not all, Skills do.

Skills displaying a Blue Damage Number do more damage as Skill Power Increases. The amount of the increase varies by Skill.

Skills displaying an Orange Damage Number do more damage as Basic Damage Increases.

Skills displaying a Green Damage Number do more damage as the Level of that Skill Increases.
So, always keep in mind these colors while equipping mods, if you see orange damage in skills, you need to increase basic damage that means you need to Equip BD mod.
Important Tips
 Shield mod upgrade will help in gain hardy at the start of the wave if paired up with Baymax
Cheshire Cat
Summary
Stats
 Armor  18231
 Reality  100018
 Normal Crit  183
 Fantastic Crit  251
 Evasion  228
First Mod should be definitely Basic Damage & attached Fantastic Crit Upgrade, as Cat’s fantastic crit is at 251 once all badges maxed on S21, so there is 91% chance of crit. With the mod upgrade, raise this stat to at least 260 ( more the better ) so that cat can have a 100% chance of doing fantastic crit, DO NOT USE NORMAL CRIT UPGRADE
Second Mod should be HP mod with armor upgrade as Cat’s HP is high & armor is quite low so definitely you need to add provide ARMOR to keep CAT alive in battle, No need for reality upgrade
Rest two mods now will depend on the team you are facing, so let’s assume the opponent team have minnie, baymax, kristoff who have modded their skill & disables can’t be evaded by Cat, cat’s evasion is 228, So if Minnie’s green skill modded & maxed it will be at 230 level, then Cat can’t evade charm or freeze in case of baymax (olaf disk), So, trigger Head Toss before Minnie’s charm or Kristoff ice slide or similar other skills & for faster Head Toss, add attack speed mod upgrade as 3rd Upgrade.
Now, the last mod should be SKILL Mod with Green Skill Upgrade ( no need of another skill upgrade) )to increase the damage of Head Toss, Now let’s say you don’t have sufficient mod upgrades for fantastic crit or attack speed, you can attach BD mod and attached Crit or speed boost as per your requirement. You can get 10% speed by using two BD damage and using only 30 upgrade pieces else you have to use 55 pieces on one BD.
So my suggestion will be
Balanced Cat
Basic Damage  Healing Power  Skill Power  Basic Damage 

Fantastic Crit  Armor Only  Head Toss +40  Attack Speed +10 
High Damage Cat
Basic Damage  Healing Power  Skill Power  Skill Power 

Fantastic Crit  Armor Only  Head Toss +40  Head Toss +40 
Faster Cat
Basic Damage  Healing Power  Basic Damage  Basic Damage 

Fantastic Crit  Armor Only  Attack Speed +10  Attack Speed +10 
Disgust
Summary
Stats
 Armor  28537
 Reality  54311
 Normal Crit  199
 Fantastic Crit  230
Disgust is OP because of her green skill damage at 160 it is 405128 & at 240 level it is 1701954. Initial cooldown is 4 seconds
First Mod should be HP mod with armor upgrade as Disgust’s HP is high & armor is low so definitely, you need to add provide ARMOR, reality upgrade is not necessary as she gets reality from red skill. I would suggest pairing her with a toon who can provide reality just to be on the safe side.
Now with disgust it is quite interesting as to save from minnies charm or freeze from wellmoded skill toon, needs to activate her Green skill faster, so you need attack mod but the interesting thing is not many players have perfectly modded skills.
So, just do a fantastic crit upgrade as it is low for her  230 & for 100% crit chance need this stat at 260, so Second Mod will be BD with Fantastic Crit Upgrade.
Third & Fourth Mod will be skill power mod with green skill upgrade to deal an insane amount of damage in the first 4 seconds
So my suggestion will be
Damage Booster Disgust
Basic Damage  Healing Power  Skill Power  Skill Power 

Fantastic Crit  Armor Only  Green +40  Green +40 
Angel (In Progress)
Summary
Stats
 Armor  38901
 Reality  64314
 Normal Crit  194
Now at a present level game if you see most of the heroes do fantastic damage, gone are the days of mulan, so you should build reality mods on toons but you never know about new toons.
Angel, have low armor & high reality once badges are maxed.
So First Mod will be HP with armor upgrade to save from normal damage hitters.
Second Mod will be BD , now none of the skills do normal damage so no need to put normal crit upgrade. One good point is that angel’s basic damage is good & enhanced by skills & disc & all basic damage is normal damage unless mentioned clearly. So, you can go with crit upgrade but I think it will be a waste of resources as getting upgrades is difficult in game also. Best shot is to give Attack Speed upgrade to mod, it will give attack boost.
Now angel’s charms is perfect so it can be evaded or reduced, so no need to work on Green Skill, I would definitely give purple mod upgrade if you are planning to use angel in silence team as it will boost her Basic Damage, at 230 level she gains 42120 BD per silence & with attack speed boost she can do good damage with basic attack.
Third Mod will be Skill Mod with Purple Mod Upgrade only if you are using angel in Silence Team
Fourth Mod will be BD mod to Spam Basic Bamage & in this case go for Attack Speed upgrade
If you don’t want to use Attack Speed, then use NORMAL CRIT BUT USE ON BOTH MODS to reach 260 by using less upgrade pieces.
Silence Team
Basic Damage  Basic Damage  Skill Power  HP 

Nor. Crit or Attack  Should be Same  Purple +40  Armor 
Normal Team ( still in progress  need suggestions )
Belle
Summary
Stats
 Armor  39534
 Reality  32502
 Normal Crit  252
 Evasion  273
 Conservation  30%
First Mod should be BD with normal crit mod upgrade, belle have already good stat on Normal Crit so definitely normal crit, so need only +8 to have a 100% chance of normal crit damage. Also, it will increase basic damage which will help blue skill as Orange Damage can be increased by increasing basic damage of that toon.
So Second Mod must be a skill power mod with a blue skill upgrade as it does normal damage.
Now, as long as Belle will be alive in battle, she can provide hardy with every basic attack so go Third Mod should be HP mod with Reality Mod as the current game is filled with OP toons with fantastic damage.
Belle’s Purple Skill can increase her speed up to 75% & in the hardy team, it can be achieved from the start of the battle & with every 3rd basic attack Belle gives hardy to ally so faster the belle more hardy & other toons can be spammed, Fourth Mod will be BD with attack speed upgrade, BD will increase blue skill damage also.
Mod Matrix
Basic Damage  Basic Damage  Skill Power  HP 

Nor. Crit  Attack Speed  Blue  Reality 
Note: Use toons who can give armor to Belle or in the later game when Normal damage OP toons will come, give armor mod upgrade
Slinky  165 R6
Summary
Stats
 Armor  68240
 Reality  150662
 Normal Crit  189
Slinky does only fantastic damage so normal crit doesn’t make sense here & the role of the slinky is to keep the team alive by spamming shields.
His skills & powerline disc provide a good amount of reality to himself & to allies when shielded. So, his reality will always be enough for him.
Therefore, First Mod should be HP mod with Armor Upgrade to save from normal damage, I know dog already has a good amount of armor but the more the better for the team.
Second Mod, should be again HP mod with Shield Upgrade, so that dog can get instantly more armor & reality from purple skill. This can also be spammed by Baymax red skill as well or some other toon as well  I will update later on this.
Now, slinky will be great for a stall team in arena & War, so just give him as much as reality & armor from purple skill, so for a good stall team Third & Fourth Mod should be Skill power mod with Purple Skill Upgrade
Mod Matrix
HP  HP  Skill Power  Skill Power 

Armor  Shield  Purple +40  Purple +40 
Another Suggestion
HP  HP  Skill Power  Skill Power 

Armor  Shield  Purple +40  Blue +40 
Minnie Mouse  165 R6
Summary
Stats
 Armor  50929
 Reality  68448
 Normal Crit  220
Minnie’s green & purple skill is great but it fails on Cat, slinky basically toons with high evasion & high tenacity. See stats explanation at top of Guide.
So, the First & Second Mod must be a skill mod with Green skill & Purple skill upgrade, so that toons with high evasion can get charmed. +40 level is max so if you want that charm should work every time Third & Fourth Mod should be the same so that green & purple skill be at +80 level if mod maxed.
But, as per my suggestion +40 is enough as it works 4 times out of 5 for me, so Third & Fourth Mod should be HP mod with Armor & Reality upgrade.
At, current game state you need more reality so you can ignore armor mod & go for Basic Damage as Fourth Mod with Attack Speed upgrade
Mod Matrix
CHARMER Minnie
Skill Power  Skill Power  Skill Power  Skill Power 

Green +40  Green +40  Purple +40  Purple +40 
BALANCED Minnie (Beast Minnie)
HP  Basic Damage  Skill Power  Skill Power 

Reality  Attack Speed  Purple +40  Green +40 
Minnie with Resistance
HP  HP  Skill Power  Skill Power 

Reality  Armor  Purple +40  Green +40 
King Louie  165 R6
Summary
Stats

Armor  55364

Reality  92816
Well KL has a good amount of reality & armor already ( armor on the lower side )
I would say all Four Mods to Skill Power Mods with Purple Skill Upgrade, no need of thinking much in his case.
If you still think of an armor upgrade as it is on the lower side, go with one HP Mod with armor piece & the rest 3 purple skill mods.
I would still suggest the 4 Purple skill mod as it will deal an insane amount of damage at the start of battle & it works really well with animal disc maxed. One banana will deal around 5664902 damage if all 4 mods are maxed +40. Source: wakkatu
Mod Matrix
Skill Power  Skill Power  Skill Power  Skill Power 

Purple +40  Purple +40  Purple +40  Purple +40 