Faith Hill and Tim McGraw are a couple in perfect harmony. The country singers (left, in 1996; right, in 2011) first saw sparks when playing together on the appropriately named Spontaneous Combustion Tour in 1996. By the end of the tour, they were engaged, and by the end of the year -- married! Since then, the first couple of country has produced three daughters -- Gracie, Maggie and Audrey -- and some wonderful duets, like 1997's "It's Your Love." But Hill maintains that their marriage works because they make the extra effort. "You can't sit back. My husband and I have made the choice that our marriage is the most important thing to us," she's said. And if anything, the union has made them both more successful: their 2006 Soul2Soul tour -- planned around their daughters' school schedules! -- was the highest-grossing country tour of all time.