These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Arkham Horror
Anticlimax Boss: Bokrug is, with difference, the easiest Ancient One in the game. His Doom Track is relatively forgiving, and if you're diligent killing monsters and closing gates you can get rid of the four Beings of Ib before he awakens, which leaves him with very little means to screw you over in a significant manner. Additionally, one of his Sinister Plots makes him turn around and go back to sleep, ending the game in (an admittedly anticlimactic) victory for the Investigators.
Yig has been statistically one of the easiest to defeat Ancient Ones, even between different version of him.
Demonic Spiders: The Hound of Tindalos; it's one of the few monsters that can attack you in any location, can move anywhere on the board, and if playing a low sanity character (poor Monterey Jack), it's almost impossible to fight as it will just nuke your brain instantly.
Plain ol' ghosts from the main game also count. They're immobile, not to mention immune to physical attack and have hefty penalties on Will, Fight and Sneak checks. In the wrong place against an ill-equipped character, they are a formidable roadblock.
Dholes. Resistance to both stats, lethal damage, high stats and toughness...the only thing keeping you sane is that it moves normally, meaning you can generally walk around it.
That One Boss: Great Cthulhu himself. His cultists effect your sanity like most other monsters. Everyone permanently loses a point of maximum health and sanity with him in play. In the final battle, he regenerates a doom token after every attack.
Atlach-Nacha is the most feared Ancient One in the game, and for good reason; sealing gates, the main method of defeating the Great Old Ones, barely inconveniences him at all. He is also extremely difficult to defeat once he awakens (though not impossible like Azathoth).