Ahh... The Mr Meeseeks episode. Whenever I'm trying to convince someone to start watching "Rick and Morty" I recommend they start with this episode.
After another adventure of trauma for Morty (Justin Roiland) he and Rick (Justin Roiland) agree on bet, Morty choses the next adventure and gets to lead, but if it's disappointing he never complains again. Their quest involves a fantasy world, Giants up a beanstalk, a criminal trial and a horrible bathroom assault. However, before they leave, Rick introduces the family to an invention of his, the Meeseeks box. You press the button, a blue character appears and completes a task you ask him to undertake, then blinks out of existence. All is going well, until Jerry (Chris Parnell) sets them an impossible task.
This is a great episode. I use it to get people to watch the show as it's a perfect example of a "Rick and Morty" episode. It introduces a regular science fiction / fantasy trope - that of a wish granting genie - but taken to an extreme by the reality of that sort of situation. It's an easy idea to get your head around, as an introduction to the Rick and Morty style. Plus, it has the bathroom rape plot point, which is probably the most horrific scene in the shows history, so if they can get over that, they're golden.
It's the first episode where you'd consider Rick and Morty's aspect of the episode as the B-Story, with the rest of the family and Mr Meeseeks being the main and most memorable sections. Justin Roiland provides the voice for the various Mr Meeseekses and it's an amazing, obnoxious, but memorable character. All of the characterisation is getting ironed out now, with Rick's belching at around the right level and a first glimpse of protection, from a man who risks his Grandsons life regularly and his mental health constantly. It gives us another insight into Beth and Jerry's failing marriage too.
Hard to credit that the episode is already six years old, but not too early to call it a classic.