Based on Bill James' Game Score stat 
Quote:
Game Score is a metric devised by Bill James to determine the strength of a pitcher in any particular baseball game. To determine a starting pitcher's game score:
Start with 50 points.
Add 1 point for each out recorded, so 3 points for every complete inning pitched.
Add 2 points for each inning completed after the 4th.
Add 1 point for each strikeout.
Subtract 2 points for each hit allowed.
Subtract 4 points for each earned run allowed.
Subtract 2 points for each unearned run allowed.
Subtract 1 point for each walk.

Cain's perfecto (101) was tied for the second most dominating 9 inning pitching performance since 1920  behind only Kerry Wood's 20 K, one hit shutout (105) and tied with Nolan Ryan's 16 K/2 BB no hitter and Sandy Koufax's 14 K perfect game.
http://espn.go.com/blog/statsinfo/po...hsweetmorrow
Code:
Highest Pitcher Game Score in 9Inning Game
Since 1920 (LiveBall Era)
Opponent Game score
1998 Kerry Wood, Cubs Astros 105
1991 Nolan Ryan, Rangers Blue Jays 101
1965 Sandy Koufax, Dodgers Cubs 101
2010 Brandon Morrow, Blue Jays Rays 100
2004 Randy Johnson, Diamondbacks Braves 100
2002 Curt Schilling, Diamondbacks Brewers 100
1973 Nolan Ryan, Angels Tigers 100
1972 Nolan Ryan, Angels Red Sox 100
1960 Warren Spahn, Braves Phillies 100