The wonderful thing about SG1 is the great variety, which makes this simple question kind of overwhelming.
There are the slam bang action episodes, which are about as about as substantial as potato chips but nearly as much fun. Most all of the midseason and season cliffhangers/openers, like The Lost City or The New Order or Full Circle/Fallen/Homecoming or Summit/Last Stand are quite fun.
But SG1 is essentially a comedy which is why a flat out comedy episode like Window of Opportunity probably is the best single episode. Other great comdies are Urgo, 1969, The Other Guys and the one that was robbed of a Hugo, 200.
Then there are the character driven episodes. For Jack, there are A Hundred Days, Solitudes, Learning Curve and Abyss. For Daniel, there are Torment of Tantalus and Forever in a Day. For T'ealc, there is perhaps the very best character episode in ten years, The Changeling. Cameron Mitchell had Collateral Damage. Even Jonas Quinn got Prophecy. (No, Carter didn't get a really good character episode. Amanda Tapping had to get another series to really stretch. Ascension was the closest, I suppose.)
Then there are the terrific weird, campy episodes. Stuff like Hathor, Holiday, Brief Candle, Prometheus,The Ties that Bind.
There are some serious political type episodes. The Unas trilogy, The First Ones/Beast of Burden/Enemy Mine. The Icon/Ethon duology. The Other Side. Crusade.
There were the episodes designed to pacify Michael Shanks, who is apparently a terminal diva, but they were pretty good. Lifeboat, Legacy, Absolute Power.
And last, there were even some nifty little sf stories. Learning Curve, Revisions, A Matter of Time, 2010.
Pick the category that seems most appealing and try the episodes in it.