Mariano Rivera stood head and shoulders above eveyone else, but looking at the top ten pitchers in relief ERA you find three pitchers from the World Champion Chicago White Sox.

Name	Team	ERA	Saves	Holds
Rivera, Mariano	NYY	1.38	43	0
Wagner, Billy	Phi	1.51	38	0
Street, Huston	Oak	1.72	23	0
Turnbow, Derrick	Mil	1.74	39	2
Cordero, Chad	Was	1.82	47	0
Linebrink, Scott	SD	1.83	1	26
Dempster, Ryan	ChC	1.85	33	0
Cotts, Neal	CWS	1.94	0	13
Politte, Cliff	CWS	2.00	1	23
Hermanson, Dustin	CWS	2.04	34	5
*Note: Pitchers with 50+ innings in relief in 2005

The first chart was ranked by ERA, but what if we look at opponent on-base plus slugging percentage (OPS)? Here's the ranking by OPS:

Name	Team	ERA	Saves	Holds	OPS
Rivera, Mariano	NYY	1.38	43	0	0.465
Wagner, Billy	Phi	1.51	38	0	0.494
Howry, Bob	Cle	2.47	3	29	0.515
Cotts, Neal	CWS	1.94	0	13	0.527
Wise, Matt	Mil	3.36	1	10	0.530
Nathan, Joe	Min	2.70	43	0	0.534
Street, Huston	Oak	1.72	23	0	0.535
Heilman, Aaron	NYM	2.18	5	5	0.542
Farnsworth, Kyle	Atl	2.19	16	19	0.544
Reyes, Al	StL	2.15	3	16	0.550

It's still Mariano Rivera, with Billy Wagner number two again. Bobby Howry's superb season comes to light in this chart. His 29 holds were third in the American League (behind the 33 turned in by Tom Gordon with the Yankees and Scot Shields with the Angels).