This post is a part of the #RPGaDAY series for 2017 by David F. Chapman and RPGBrigade. For more information, see this post at AUTOCRATIK. I'm modifying per suggestions from S. John Ross as well as applying my own interpretations. Comment with your answers or links to your own posts!
Day 1 - What published RPG do you wish you were playing right now?
For the sake of honesty, I'll first reveal the true answer, which is "all of them." That having been said, for the sake of having something to write about, my choice for the more practical answer is Blades in the Dark by John Harper.
For my own peace of mind, I decided not to name a game that I've already played, because I am all about trying new games. Blades was the first title to come to mind, mostly because I'd just recently finished my read-through of the rules, and because I also recently listened to an episode of The Misdirected Mark Podcast dedicated to a particular piece of rules tech from Blades, Position and Effect. The game is about portraying a criminal group in a fantastical Industrial Revolution-era city and trying to advance your crew from petty criminals to major players in the city's underworld. I'd call it a gritty game; the tone is certainly dark (e.g., the title), and the game is such that there is danger at every turn. Characters are living a constant struggle against the law, other criminal organizations, and their own personal vices. I, personally, consider it to stretch the definition of Powered by the Apocalypse (PbtA), but regardless, its design roots in Apocalypse World are unmistakable. Because of that, it takes advantage of a modern, strongly narrative system, yet still offers the challenge and excitement of seeing a party of low-level pipsqueaks grow into a powerful cadre of badasses. This is another reason I wish I were playing it right now: it would probably mean that I'm involved in a multi-session campaign, which is a hard thing to make time for these days.
Something that jumped out at me in considering this question and this game is that I specifically want to play Blades in the Dark, as opposed to run it. Similar to AW, Blades is low-prep for the GM and a "play to find out what happens" type of game, but the system contains a lot of moving parts for the GM to handle, so it's a lot harder to just pick-up-and-run than it is to pick-up-and-play. I should note that this is not a distinction that I often consider; I subscribe to the school of thought that the GM is another player in the game, and if another title had come to mind when I asked myself this question, it probably wouldn't have mattered so much if I wanted to play the game or run it. Blades is an outlier, though. Most PbtA games perhaps ask the GM for rules familiarity (not even necessarily mastery) but not much in the way of campaign prep before the game starts. Blades has a lot going on, though, and the GM has to be prepared to keep track of a lot of both short- and long-term processes as play progresses. I don't think this is a bad thing, I just think if this question asked specifically about running a game, I may have had a different answer.