To me, two guys stand out: Edmond O'Brien was one of the very early performers of the Johnny Dollar role and does a good job. Bob Bailey took over near he end of the series (1955-60) and his time is the one I want to focus on here.
Working for the Universal Adjustment Bureau, Dollar was an insurance investigator. This immediately made him different than detectives on other detective shows. The show revolved around his expense account and it showed us that Dollar was honest - again, most detective shows on the radio employed less-than-honest gumshoes. Dollar was up front with his doings and he told us about everything he spent his money on.
Other than O'Brien's stint, Bailey was the absolute class of the show. While the earlier versions of Dollar are hard-boiled, noirish detectives (again - like the majority of radio detectives at the time), Bailey changed the way we look at Dollar. Bailey turned Dollar into a real persona; though he was just as tough as his predecessors, he also made the character caring, bright, an avid fisherman and a regular guy. Bailey not only breathed new life into the role of Dollar but he seemed to do it so effortlessly; he was simply natural doing what he was doing - as if he were made for the role.
The stories seemed to become better and more varied as well when Bailey was Dollar. Breaking the plots away from the normal detective program, Yours Truly, Johnny Dollar did some different things to set itself apart from the others while Bailey was at the helm.
For one thing the show dealt with insurance rather than murder - something that all of the many detective shows were doing. Yes, there was murder involved sometimes on YTJD but that wasn't what every show was about. The series also dealt with then-current and timely issues -- the space race for example and the Cold War. These little touches and Bailey's perfect portrayal make the Bailey years some of the finest radio listening you'll ever come across.
Another different thing when Bailey was on the show is that there are several 75 minute serials that were sustained - no commercials - therefore, we got a very complete picture of the story being told.
For just the years when Bailey was Dollar, I'd give the show 4 stars - my highest rating.