There are a lot of great options, but the easiest and best choice is closer Lee Smith. When it comes down to it...to be an elite closer for as long as Smith was...should be enough to get him in. For 14 years he held the all time saves record with 478. Trevor Hoffman broke his record (which now belongs to Mariano Rivera). The best part about Lee Smith is that he did it on 8 different teams...and did it well. This guy passed the eye test. Ask any real baseball fan during the 80s...who the best relief pitcher was and they would say LEE SMITH.
One endearing quality is that he played the majority of his career on the two most tortured franchises in history...the Cubs and the Red Sox.
There are only 5 pitchers in the hall based solely on being a reliever...that needs to change. The 7 time all-star should be in the hall of fame. Period.