I'm interested in the opinion, especially now that we have a large pool of AO's and the like to choose from, what the most captivating and thematic Old One that really draws you into the story.

I ask primarily after a 6 player game, against Cthulhu, where only I and one other person were familiar with the game played. We had 4 Newbies, and gave a very in depth tutorial on the game before we started. The first turn was rocky, with the typical "what should i do" and "so is that is for me?" questions, and Cthulhu was relentless with the mysteries that forced us into the sea, as well as the Madness From the Sea Rumor on the First Mythos. We were able to complete the first Mystery, but only shortly before all investigators went insane at the same time.

It was an ass kicking that i thought left a bad taste in the new players mouths, to which, instead, was surprised by the enthusiasm as the players felt the theme was top notch and really gave that pressing feeling of going insane and trying to keep our sanity high in conjunction with the mass amounts of Hallucinations, Paranoia and other Madness cards being passed around.

i felt, in this case, that the theme really stuck out, and the players figured facing Cthulhu was as hard as it should be. 3 of the 4 new players wanted to play the game again the next night.

In terms of theme, which AOs stick out the most and really pull you into their story?

Well of all AO so far Abhoth is by far the most disgusting of all possible big enemies, not only is the AO card pretty graphic but the cards are written in a language that creates such an aura of rotting, decay and horror that I found it especially weird (and of course interesting) fighting the "source of all uncleanliness".

That doesn't mean of course that I consider the other OAs less interesting or thematic convincing, Abhoth is just especially filthy and abominable.

BTW I would like to know what game card or game play element led to this wipeout of the group? A rumor unsolved going off?

yeah if i recall correctly, it was the "every investigator loses x stamina & sanity unless they discard a blessing" mythos card.

i play with easy removed.. so there's never respite from the mythos deck.

edit: and i must agree, Abhoth adds a nice.. disgusting touch, with her creature encounters. . i feel she is a tad too easy though, and i actually change her mythos deck because of this.. i replace a few of the green mythos with yellow, in each stage (it's blasphemous i know).. but the only thing that makes her easy is the fact her Reckoning will barely ever make a difference and pressure the investigators.

That's a mythos card I never experienced so far, sounds however like a real teamkiller, I mean how many investigators have blessings

in an average EH game? 1, rarely 2, so far I never had a group with 3 or more, because EH doesn't have a location where you practically get a guaranteed blessing (South Church in AH).

Back to your question in the OP. I also consider Nephren-Ka also a thematically strong AO because an entire extra board is practically dedicated to the fight against him.