That may not be useful to a lot of other people as a recommendation, but No Thank You, Evil! and Hero Kids are both designed with shorter scenarios and challenges in mind, due to their target audience. I don't think I've ever run NTYE! for more than an hour at a time. Outside specific games, I'd probably focus on lean games with simpler character sheets, like Risus or Fate Accelerated. FAE + It's Not My Fault would work well.
But… I mean… Almost all my weekly Roll20 gaming is comprised of 2 to 2.5 hour sessions, so… pretty much any system, I guess, if you're talking about being able to play multiple short sessions.
ALSO, any game where everyone playing is on the hook to come up with a lot of stuff out of their head, all the time, is probably best confined to shorter sessions. I find people lose their inventive steam after a few hours, so if you want to keep things popping in games where the players have to invent a lot of stuff on the fly, I'd recommend shorter sessions.
Who needs therapy if you've got "impactful" RPGs, right?
Impactful is such a weird word. I've had lots of memorable sessions. Impactful seems like "something that changed you outside the game" and I'm not convinced that's why I play RPGs, so…
Man, I dunno.
Okay, here's one, maybe.
Dogs in the Vineyard. Our second serious fight. We'd already had one smaller altercation earlier in the game, so the players had seen how the conflicts worked, and more to the point they saw how FALLOUT worked.
Second conflict, things started getting heated, and someone pulled out a gun.
And the players – some experienced, pretty seen-it-all gamers – kind of pulled back from the table and were like "Whoa. Shit. Hang on a sec."
That game made guns fucking scary. They made them as goddamn dangerous as they are, you know?
(Best of all, the way fallout worked, you got fights where you didn't really know how fucked you were until the shooting stopped.)
Dogs remains the only game in which the players (and, by extension, the characters) reacted to someone pulling out a gun the way real people would, and I (obviously) remember that, to this day.
* Monday: Pirate 101 with Sean and Kaylee. (Alternately, family game of Masks.)
* Tuesday: Normal Tuesday Roll20 group, playing whatever we're currently playing.
* Wednesday: Overwatch with Kate and Kaylee. (Alternately, family game of Masks.)
* Thursday: Dungeon Crawl Classics, GMed by Tim.
* Friday: Playing DnD 5e Adventurer's League, via Roll20/Discord.
* Saturday: Luxurious full-length afternoon of DnD 5e or Masks with Sean, Kate, and Kaylee
* Sunday: The X-wing combat game with the kids.
This one's a bit easy for me: Tales from the Loop.
It's entirely possible this answer is chicken-egg cheating, since Tales from the Loop started life as an art book before it was an RPG, but regardless, the cover's a perfect match for what the game's about.
As an added bonus, it's a solid game that'd I'd really like to get to the table. (I heard it's the same system as Mutant Year Zero, but I haven't read that – the basic mechanics are very close to Blades in the Dark, and it leans hard in the direction of pbta-style conflict resolution.)
Tales from the Loop
One of the nice things about recording all our online game sessions is it makes it pretty easy to simply look this kind of thing up. (I knew the answer, though I was curious about the specific numbers.)
The 'winner' in this case is a apocalypse engine hack I built for our Star Wars game (a campaign that went through three rules systems over the course of 30+ sessions). My hack (here: https://www.dropbox.com/s/fwnq7ok585thk1c/SWW%20Rebel%20Ops.pdf?dl=0) is a rebel-era adaptation of Star Wars World, which in turn owes DNA to Urban Shadows, I think (understandable, since SWW was written by Andrew Medeiros). Since August, I ran the game eighteen times. That game wrapped up at the end of April.
Aside from that:
– Ran a six-session Dungeon World adaptation of Deathfrost Doom, plus a couple asynchronous DW adventures with Kaylee, via Google+ polls (http://randomaverage.com/index.php/2016/08/dungeon-world-with-kaylee-via-google-polls/), so call that eight sessions.
– Ran a couple sessions of Lady Blackbird (someday I'll run a game that makes it further than escaping the Hand of Sorrow).
– Ran four short sessions of a World of Dungeons: Breakers/The Secret World mashup, just recently.
– Played (actually played) in a couple sessions of DnD 5e (Adventurer's League), and ran a couple sessions for my family. Solid game.
– Played (actually played) in a Dungeon Crawl Classic 0-level "funnel" game. (You can't spell funnel without FUN.)
– Ran at least one or two games of No Thank You, Evil! with the kids.
So: That's ~36 game sessions since last August, with 50% going to the Star Wars Rebel Ops pbta-hack. Not too shabby, really.
SWW Rebel Ops.pdf
Shared with Dropbox