''Happy families are all alike,'' wrote Tolstoy. ''Every unhappy family is unhappy in its own way.'' Too bad Leo never met the Ewing clan; they could have given him story material that would have curled Anna Karenina's hair. Love them or loathe them, when Southfork's residents were at their thriving, conniving best, you couldn't turn your back on them without becoming a pop-culture outcast. When Dallas first flickered across TV screens on Sunday, April 2, 1978, nobody could have imagined it would become the signpost of an era a long, lewd, delectably lurid family feud, megadosed with sex, money, and cattle, that both foretold and outlasted the Age of Greed and provided the most abidingly awesome villain in TV history. Surely the Smithsonian, which houses such tube talismans as Fonzie's leather jacket and Archie Bunker's wing chair, can find a way to display Dallas' most durable image: J.R. Ewing's 10-gallon oil slick of a grin.
On May 3, after 356 installments of the kind of behavior that gave new meaning to the phrase ''Texas crude,'' the Ewings will become history. On these pages is our tribute to Dallas: the greatest moments, the best lines, the faces whose names you never knew, the plot twists that still have viewers scratching their heads, and the ways we'd like to see J.R. hang up his hat. Not forever, we hope surely sometime he'll have the last, nasty laugh.