Based on 340 battles ranging from Bronze to Gold April S2, these are the ones I usually encountered or used. [No summoners and Legends included]

TOP 10 COMMON

Goblin Psychic - chaos
Deeplurker - chaos
Disintegrator - chaos
Chaos Agent - chaos
Failed Summoner - untamed
Sand Worm - reward
Venari Scout - chaos
Hill Giant - chaos
Supply Runner - chaos
Radiated Brute - chaos
Flying Squid - chaos

TOP 10 RARE

Venari Crystalsmith - reward
Xenith Monk - chaos
Regal Peryton - chaos
Shieldbearer - untamed
Furious Chicken -reward
Mycelic Slipspawn ₥ - chaos
Cursed Windeku -chaos
Twilight Basilisk - reward
Pelacor Arbalest - chaos
Tenyii Striker -chaos
Time Mage - chaos

TOP 10 EPIC

Uraeus -reward
Djinn Chwala - reward
Prismologist - chaos
nter - chaos
Phantom Soldier - reward
Halfling Alchemist - promo
Lava Launcher - reward
Forgotten One - chaos
Lobstradamus - untamed
Djinn Renova - reward
Magi of Chaos - chaos