The old adage that defense wins champions has been thrown around for years, but there is actually legitimate information to back that up.

Five of the last seven national champions have ranked in the country's top 25 with regard to opponent offensive rating, and while the best defensive teams don't always reach March Madness on defense alone, it's a skill successful squads must have. In turn, it's something you need to keep an eye on when filling out brackets.

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West Virginia Mountaineers' success over the past few seasons has been due to their strong defense and ability force turnovers -- they rank fifth in the country in defensive rating this year, allowing 91.2 points per 100 possessions.

A ton of credit for their highly effective defense should go to their press, which is the main reason why they're the nation's best team in turnover percentage, forcing opponents to turn the ball over once every four possessions. Their opening matchup should play right into their hands, as Bucknell likes to play fast and has the propensity to be forced into turnovers.

4. South Carolina Gamecocks - 7 Seed, East Region

With the increased national emphasis being put on three-point shooting, a common theme of good defenses is limiting success beyond the arc. That is something the South Carolina Gamecocks do better than any team on this list, ranking fifth in the nation by holding opponents to just a 29% success rate.

But if there's one places to get after the Gamecocks, it's from the free throw line -- they rank last in the field in free throws per field goal attempt.

However, to spin that positively, South Carolina will not let opponents get anything easily. Their 64.5 points allowed per game ranks within the top 30 in the country, and a first-round clash with Marquette will be an interesting one since the Golden Eagles lead the country in three-point percentage (43%).

3. Virginia Cavaliers - 5 Seed, East Region

Another mainstay on this list is the Virginia Cavaliers, who, under head coach Tony Bennett, has built a program out of grinding opponents down defensively.

This squad uses the country's slowest pace to limit the number of possessions per game, along with putting the clamps down in the half court. As a result of this slow pace, Virginia leads the nation in opponent's points allowed per game (55.6).

Furthermore, once they slow opponents down, they will not allow good looks, leading to them ranking 13th in opponent's field goal percentage and 26th in opponent's three-point percentage. Their foe in the first round, UNC Wilmington, will be a good test -- they rank 74th in the nation in pace and 3rd in offensive efficiency.

2. Wichita State Shockers - 10 Seed, Mid West Region

Head coach Greg Marshall has once again gotten the Wichita State Shockers in a position to make a surprise run, thanks in large part to the nation's second-ranked defense.

The Shockers are fourth in the country in opponent's effective field goal percentage, meaning they do a good job of limiting quality three-point attempts. That point is further driven home since they allow teams to shoot a shade under 31% from three, which ranks 17th overall.

Wichita State also uses their highly efficient offense to put pressure on other teams to put up points in order to not get left in the dust. Their defense will be tested against Dayton, who shoots just under 39% from behind on the arc this season.

1. Gonzaga Bulldogs - 1 Seed, West Region

The Gonzaga Bulldogs have ridden their top ranked defense to a 32-1 record and the 1 seed in the West region. Similarly to Wichita State, Gonzaga's strong defense is aided by their efficient offense -- ranking in the top 10 in both offensive and defensive efficiency -- but the Zags do it a little better on both ends of the court.

They rank 7th in the nation with regard to points allowed behind a defense that limits opponents to shoot 40% from two and 30% from three, both ranking in the top 8 nationally. They also don't help opponents get any free ones, as they're one of the nation's top 10 teams in free throws per field goal attempt.

A truly great defense is one that makes life hard on opponents in all aspects of the game, and that's quite accurate when talking about Gonzaga.