Which sixth episode of a season is your favorite?
It's easily "Ask Jeeves" for me, though I'm quite fond of many of the other episodes listed--"No Exit" and "Red Sky at Morning" are both fun examples of Sam and Dean having to deal with a third party butting in; "Yellow Fever" is a classic example of a seemingly goofy episode going hard off the rails into horror; "You Can't Handle the Truth" has Dean finally twigging to just how wrong things are with Sam--
But "Ask Jeeves" I watched 8 times in 1 week after it aired. I love everything about it--the glazed doughnut coffee, the various references to Clue, Dean peering into the empty suit of armor, Sam awkwardly claiming lactose intolerance, his easy assurance that he and Dean like each other (because they do!). And then at the end of all the silliness and fun, Dean's shocking brutality as the Mark of Cain starts to make itself known again. Just excellent stuff all around.