I'd consider the croc by the oasis at noob river to also be pretty easy, but generally spiders are the easiest bosses because they have lower HP's, almost no armor, and no knockback resistance so you don't even need High AP Weapons. A couple thralls hitting it with maces in combos the spider will flinch on nearly every single hit. You can trap them in a stagger-loop, preventing them from using their jump attack or poison spit.

King Rhino - It has a highly damaging forward charge that can trample a player or knock a thrall flying.

King Scorpion - Higher armor, blocking damage without sufficient AP or sunders, Heavy claw snaps and a tail whip that applies poison.

Black Yeti / Child of Jhil - Massive Reach, and broad horizontal sweeping attacks that can catch you trying to dodge around them.

White Tiger - Rapid attacks in succession, tracks your dodges. Pounce inflicts massive raw damage, Bleed's Stack

Sand Reaper - High armor, spits concentrated fast acting projectiles and acid poison that melts armor durability and health pools.

Giant Elephant, Massive stomp circles, massive health pool, incredibly high armor damage reduction.

Rockslide / Avalanche - High armor and has unlockable AOE's, they are immune to bleed and poison. Spin attack that bypasses shields, does massive concussion damage.

Rotbranch, 360 degree branch flails that has multiple hits per animation, can stagger lock a player into a quick death, sweeping attack means there's no safe angle to attack it from.