Thematically my favorite is Hastur. I also love the King in Yellow expansion & Herald!

Regarding the game mechanics I like Glaaki and Shudde M'ell best. With these two I really get the impression that things are slowly going worse in Arkham. Their 'slumbering' abilities actually evoke the feeling of terror that the terror track is supposed to measure.

I always loved Hastur and his King in Yellow and Carcossa theme.

On the other hand, simply remembering the games against the german Ancient One Eihort makes me grin like a psycho.

I love Y'Golonac. I like the fact that you have to keep shopping to a minimum or face the consequences.

Y'Golonac has really too short a doom track. (Worse than Yig, especially in large group because each shopping action seems to add 1/2 doom on average and more players -> more gearing needed) Last time we played him with the kingsport expansion (minus the Kingsport board), we decided to add a random herald.. Ghroth + One unlucky roll on Ghroth + One Double Doom + Old Journal (And we thought most of the tomes were in the Unique Item) woke him up by turn 5. One investigator got devoured by his Corruption attack (KH epic battle AO plot card) due to having 2 Call friend spell (which were never used). which brings up a question, when are we allowed to discard items? Upkeep only in which case a player can't discard to avoid taking damage since epic battle cards are drawn after upkeep. Also, are we allowed to discard items at all at the start of the final battle. (For eg. Find gate/Call Friends are useless in the final battle, can they be discard before the final battle..)

To respond to the debate, My fav. is probably Cthulhu. Makes for reasonably long game, easy to keep track of AO powers, not very likely to have investigators prepare for the final battle with him. 2nd is probably Shudde m'ell. (Note they both have 7 turns maximum on avg to defeat in the final battle and all investigators will be alive the full 7 turns, although more unlikely for Shudde m'ell)

I have a fondness for Tsathoggua and Shudde M'ell. I like Tsathoggua because he will **** your **** up without even caring, and Shudde M'ell because it's really fun to say. I also have a fondness for the mythos lore behind both Tsathoggua and Hastur.