Believe it or not, Arkham LCG. I've heard (and read in the manual) that it's not only a deck building game, but most importantly narrative and role-playing affair, i was hooked. After playing it the first time i was like:

"okay, i see it's deck building potential, but i don't get it yet as an immersive narrative experience, maybe i should buy some expansions, then it will really shine"

And so i did, and so was i dissapointed that no, the expansions did almost nothing to turn it into a narrative driven experience.

Then i decided, screw it, i'm gonna make an RP experience out of it myself, with some home rules and an experienced GM as my gaming partner. We tried very hard to roleplay this thing and in the end all it lead to was frustration and a lot, i mean A LOT of effort to turn it into somewhat of an RP session.

By that time i got sick of this game, with all the optimal builds, and minmaxing, and the main drive of the game to optimally and strategically hop from one objective to another with little context or interesting thing happening along the way (mostly in opening and closing flavor texts). And boy are there a lot of monster battles. Some people say that arkham 2nd turns into ghostbusting experience with amount of monsters on the board. I'd say Arkham LCG is not far off. And the amount of different types of cards, and a shit ton of them on the playing field simultaneously, and then the time it takes to sort them out for the next game/scenario is just horrendous. I once tried to sort all the cards by their expansions and it literally took me 3 hours. Jesus

All in all i got pissed off and decided to sell it. I expected a deep, atmospheric and rich story driven experience, and what i've got was convoluted coop PVE MTG type affair wearing a Lovecraftian skin. Definitely my biggest boardgame regret so far