20+ IP, 1 BB/9 or lower, 8+ SO/9 (Since 1900)
1989 Dennis Eckerlsey (A's): 57.2 IP, .47 BB/9, 8.58 SO/9
1990 Dennis Eckersley (A's): 73.1, .49, 8.96
2006 Ben Sheets (Brewers): 106, .93, 9.85
2008 Mariano Rivera (Yankees): 70.2, .76, 9.81
2010 Edward Mujica (Padres): 63, .86, 9.43
Edward Mujica (who pitched poorly for the Indians from 2006-08) was traded by the Indians to the Padres in April 2009 for either a player to be named later or cash. He has the best WHIP of any pitcher with at least 56 IP and his ERA (a solid 3.14), like many relievers, was affected heavily by one bad outing (5 ER in IP on August 4).
The Padres have a knack for finding quality relievers (Joe Thatcher, Heath Bell, Mike Adams, Ryan Webb), but with his amazing walk and strikeout totals, Mujica might end up being better than any of them.