Make a Mob In Only ONE Command

This system is using the Attributes and The Equipment Tag (Now ArmorItems, and HandItems in 1.9.)

Here are some steps 

1. Give yourself a command block /give <your username> command_block or comm, press TAB on your keyboard.

2. Type /summon <EntityName> <coodinates>

Here is a list of Entity Names

Drops
Item
XPOrb

Immobile
LeashKnot
Painting
ItemFrame
ArmorStand
EnderCrystal

Projectiles
Arrow
Snowball
Fireball
SmallFireball
ThrownEnderpearl
EyeOfEnderSignal
ThrownPotion
ThrownExpBottle
WitherSkull
FireworksRocketEntity

Blocks
PrimedTnt
FallingSand

Vehicles
MinecartCommandBlock
Boat
MinecartRideable
MinecartChest
MinecartFurnace
MinecartTNT
MinecartHopper
MinecartSpawner

Generic (Note: these entities are removed)
Mob
Monster

Hostile Mobs
Creeper
Skeleton
Giant
Zombie
Slime
Ghast
Silverfish
Blaze
LavaSlime
EnderDragon
WitherBoss
Witch
Endermite
Guardian

Neutral Mobs
Spider
PigZombie
Enderman
CaveSpider
Wolf
VillagerGolem

Passive Mobs
Bat
Pig
Sheep
Cow
Chicken
Squid
MushroomCow
SnowMan
Ozelot
EntityHorse
Rabbit
Villager

3. Go on the minecraft wiki

4. Search for Chunk format

5. Use the NBT Tags for your command
Example:

1.8:
/summon Zombie ~ ~1 ~ {Attributes:[{Name:"generic.maxHealth",Base:100}],Equipment:[{id:minecraft:diamond_sword},{id:minecraft:diamond_boots},{id:minecraft:diamond_leggings},{id:minecraft:diamond_chestplate},{id:minecraft:diamond_helmet}],CustomName:"Diamond Zombie"}

1.9:
/summon Zombie ~ ~1 ~ {Attributes:[{Name:"generic.maxHealth",Base:100}],ArmorItems::[{id:minecraft:diamond_boots},{id:minecraft:diamond_leggings},{id:minecraft:diamond_chestplate},{id:minecraft:diamond_helmet}],HandItems:[{id:minecraft:diamond_sword},{}],CustomName:"Diamond Zombie"}
The extra {} brackets are for the offhand slot in 1.9

You can copy and paste these commands.

6. Enjoy