You can read about the whole series of best bullpens by decade at this link.
OK, the 1990s. By now, every team had one reliever designated as the closer, and that person was pitching fewer innings than ever. The number of 30-save seasons increased from 55 in the 1980s to 131 in the 1990s. Only three of those guys pitched even 90 innings in a season: Doug Jones, 111⅔ for the '92 Houston Astros; Jeff Shaw, 94⅔ for the '97 Cincinnati Reds; and Jeff Montgomery, 90 for the '91 Kansas City Royals. Mariano Rivera pitched 107⅔ innings as a setup guy in 1996; when he moved to the closer role, he reached 80 innings just once for the New York Yankees the rest of his career.
Bobby Thigpen of the Chicago White Sox shattered the single-season saves record in 1990, going from 46 (the previous mark, set by Dave Righetti in 1986) to 57 in a single leap. He'd record just 54 saves the rest of his career, indicating the often-volatile nature of the position. Meanwhile, offense increased dramatically starting in 1993, pushing the percentage of complete games from 10 percent in 1990 to 5 percent in 1999 and the number of pitchers used per game from 3.02 to 3.56.
My three best bullpens of the decade:
1. 1995 Cleveland Indians (100-44): 32-13, 3.05 ERA, 50 saves, 74-2 when leading after seven innings
They were overshadowed by a powerful lineup featuring Albert Belle, Jim Thome, Manny Ramirez, Eddie Murray, Kenny Lofton and Carlos Baerga, but the bullpen was awesome, led by Jose Mesa, who went 3-0 with a 1.13 ERA and 46 saves in 48 opportunities. The Indians didn't lose a game they led heading into the eighth inning all season, and Mesa finished second in the Cy Young voting and fourth in MVP voting. Beyond him, the pen had Julian Tavarez (10-2, 2.44 ERA in 85 innings) and Eric Plunk (6-2, 2.67, 71 Ks in 64 innings) from the right side and Jim Poole (3.75 ERA) and Paul Assenmacher (2.82 ERA) from the left side. The Indians' 3.05 ERA ranked tied for the 10th in the decade; all but one of the others in the top 10 came in the much-lower-scoring 1990-1992 time span. (The exception being the 1995 Cardinals, an otherwise awful team that posted a 2.71 bullpen ERA, second-best in the decade.)
2. 1990 Oakland Athletics (103-59): 14-10, 2.35 ERA, 64 saves, 87-2 when leading after seven
That's right, the Nasty Boys weren't even the best bullpen of 1990. This pen had the lowest ERA of the decade -- by 0.36 runs -- as well as the lowest OPS allowed and lowest batting average. These A's were lockdown in the late innings thanks to Dennis Eckersley, who posted a 0.61 ERA with 48 saves. In 73⅓ innings, he allowed just 41 hits and four walks in perhaps the best relief season ever. After him, it was the same crew as the great 1989 bullpen: Gene Nelson (1.57 ERA, five saves), Rick Honeycutt (2.70 ERA, seven saves) and Todd Burns (3.14 ERA). Joe Klink tossed in a 2.04 ERA over 39⅔ innings. The A's would rather forget about the World Series, when they were swept by the Reds -- and blew two leads after seven innings (although one of those was by starter Dave Stewart).
As for those Reds -- yes, Rob Dibble, Randy Myers and Norm Charlton are maybe the most famous bullpen of all time, but I can't put them in my top three. Just to compare to that Oakland pen:
A's: .210/.272/.305
Reds: .233/.314/.343
The Reds had the 20th-best OPS allowed in the decade, and while they did have a much higher strikeout rate than the Oakland pen and the ninth-best of the decade, I can't get past this number: The Reds lost five games they were leading after eight innings; the major league average that season was 2.8 games lost. The Nasty Boys were lights-out in the postseason and that's what we remember, but Myers had six losses and six blown saves, and Charlton had just a 3.02 ERA as a reliever (he also started 16 games that year). There also wasn't much depth after those three.
3. 1997 Baltimore Orioles (98-64): 33-24, 3.33 ERA, 59 saves, 79-4 when leading after seven
The Yankees won the American League East every season from 1996 to 2006 -- with the exception of 1997. This Orioles bullpen was a big reason why. Baltimore lost just once when leading after eight innings; Myers posted a 1.51 ERA and recorded 45 saves in 46 chances. Arthur Rhodes and Armando Benitez provided a lefty-righty, flame-throwing setup duo and combined for 208 strikeouts in 168⅔ innings, huge numbers for the era. The ageless Jesse Orosco, then 40, allowed just 29 hits in 50⅓ innings while posting a 2.32 ERA. Terry Mathews (4.41 ERA in 63 innings) and Alan Mills (4.89 ERA in 38⅔ innings) weren't quite as strong as the long or mop-up guys, but the top four carried the day.