So I think your question is as much what is the the most typical episode that you could show someone and say 'this is Supernatural'? Obviously that is next to impossible over 10 seasons but here are the two that came to mind for me:
Scarecrow
Reasons:
Urban legend
People dying in the teaser
The boys fighting over their father
Dean being the good son, Sam being the rebel
Sam making his own decision about moving on - I don't know that it ever occurred to me before but him getting out of the Impala in the middle of the night to walk, Dean telling him that he should do what their Dad wanted and Sam refusing to do anything except what seems to him to be the right thing is probably meant to be an exact parallel of what happened the night Sam left home. (I hope this group of writers never write that flashback....)
Dean using an alias, annoying the locals by being a smart-ass, rescuing the next set of victims.
Reference to using Sam as a puppy-dog
Sam meeting a demon who wants to manipulate him.
This episode has direct links all the way back to the basis of the mythos of the show (as Meg is related to Yellow-Eyes) and all the way forward to the end of Season 2 storyline-wise - a pretty good arc.
Dean needing a researcher, sucessfully finding a good one but ... discovering that Sam is the most reliable (gets knocked out and nearly sacrificed)
Dean realising Sam is his own person and his decision making skills are actually pretty good - and both saying so and making an apology without necessarily having to say it (a skill he could relearn these days ...)
Sam reluctantly using the less honest of his hunting skills (steals car)
Sam deciding they are stronger together as a team.
Sass between the brothers.
Driving off in the Impala.
This episode has literally everything that grabbed people about the show. It is a pretty great little episode.
Then at the other end of the spectrum we have Sera Gamble and Ben Edlund and some VERY nervous actors ....
The French Mistake
Reasons:
Meta has been a part of the show for a long time.
Fans love behind the scenes stuff.
Fans know enough about everyone on the show to get about 85% of the jokes
Crew has been together so long the other 15% of the jokes are like the gag reels
'You married fake Ruby?', Locks and keys, Castiel having a storyline (!), Balthazar, Bobby's place ...
and, most of all, self-awareness (Why are we taking ourselves seriously?):
"Season 6"
I think if you wanted to introduce someone to SPN and also explain why the SPN fandom is famous on Tumblr for having a 'gif for everything' - you would probably cover most of it with these two episodes.