Being more of a mystery than a horror reader, of course I like all of the mystery squares -- as well as the squares adding diversity to the game ("Diverse Authors", "Terrifying Women", and the new "International Women of Mystery") and the squares that allow me to sneak in a Terry Pratchett book or three ("Supernatural", "Witches" / "Spellbound" -- the latter also for other reasons).
But truth be told, the squares I am enjoying the most are those calling for a specific topical reading prompt, e.g. "Full Moon", "Creepy Carnivals", or "In the Dark, Dark Woods"; as well as those calling for a specific regional or calendarial setting ("Darkest London", "Southern Gothic", "Set on Halloween", etc.). For one thing, these are the prompts that particularly showcase our bingo hosts' creativity, and for another, what always amazes me is the wide selection of books that fit these categories -- for each of them, you can go all the way from romantic suspense to the most gruesome and terrifying horror and still find something that matches the square's requirements. They're also the squares that make me take the closest looks at the books on my TBR, reading book descriptions etc. and looking for matches, which in turn increases my anticipation of the game!
Here's a compilation of my favorite squares from bingos past (in alphabetical order, regardless of year) ... added to which, I have to say that I also love every single one of the new squares MR has so far introduced in connection with this year's game. To mark the fact that yet another thing about bingo I'm truly enjoying are the group and (impromptu) buddy reads, I'm also including the "Reads with BookLikes friends" square from the 2016 bingo card -- even if group and buddy reads are by now such an ingrained part of the game that a square specifically calling for them seems highly superfluous at this point.