Just don't go too crazy with your new power
And so we bid a fond farewell to season 3....
This would be a really hard choice if I were going off of person rewatch favorites--"Roadkill" is a fun outsider POV, "...And Then There Were None" gives us some great Rufus (if only he didn't die at the end of it!), "Blade Runners" is great (Dean's agony of the defacing of Baby is brilliantly done), "Safe House" is a masterclass in editing and set design, "Ladies Drink Free" made me like Claire for a little while, and "Scoobynatural" is just plain old silly fun.
But in this case I'm going with a slightly different set of criteria: "On the Head of a Pin" is a hands-down tour-de-force in every aspect, from script to acting to cinematography. It's agonizing to watch, but that's because it's flawlessly constructed. It's also the episode that got me interested in pulled screen caps, which led to a whole bunch of friendships and fics that otherwise wouldn't have happened.