For a full list of episode titles go link.

For me it's between season 2 and 3. They has episode titles which made you wanna REALLY watch the episode.

I would choose S4 for the Monster at the End of This Book alone! But it has so many other great titles too. Are You There God? It's Me Dean Winchester, It's the Great Pumpkin Sam Winchester, Jump the Shark, When the Levee Breaks, Criss Angel is a Douche Bag, On the Head of a Pin, etc.
My second choice would probably be either S2 [for all the great song titles] and S5 [for more great song titles!]

they come up with the best titles of any show I've ever watched!!

Or S5, for some reason I really love a lot of the titles to the episodes in those seasons!

can see a nice list of titles here as well: link

i loved the 4th season for all...the tittles, the stories, the characters, hell/heaven...that was for me one of the best season of spn with the 5th....

idk

Sympathy for the Devil, Good God, Y'All, Free to Be You and Me, The End, Fallen Idols, I Believe the Children Are Our Future, The Curious Case of Dean Winchester, Changing Channels, The Real Ghostbusters, Abandon All Hope, Sam, Interrupted, Swap Meat, The Song Remains the Same, My Bloody Valentine, Dead Men Don't Wear Plaid, Dark Side of the Moon, 99 Problems, Point of No Return, Hammer of the Gods, The Devil You Know, Two Minutes to Midnight, Swan Song.