Welles' radio work has to be heard, first of all, as a breakthrough in American radio drama. Until the mid-1940's, there was nothing which had his balance of content quality with imaginative technique, and of course, until after the War, these shows were live. Because all such breakthroughs are absorbed and improved upon, even the best of these shows do not seem so astonishing as they did at the time. To listen to the programs live, at the time, with heavy effort to boost the AM signal by the big base speaker of the day, had a power and an immediacy, insome ways, unmatched in today's digital and hi-fidelity.
Besides "Dracula" and "Sherlock Holmes," which you mention, my favorite Mercury Theater pieces are "The Man Who Was Thursday," "The Pickwick Papers," "Treasure Island," "The Affairs of Anatole," "The Thirty-Nine Steps," "Abraham Lincoln," and "Jane Eyre." My favorite Campbell Playhouse shows are "Rebecca," "A Christmas Carol," "Counselor at Law," Private Lives," "Liliom," "The Green Goddess," "The Glass Key," "Twentieth Century," "Our Town," "Algiers," "The Magnificent Ambersons," and "Dodsworth."
As with his films, the quality of recordings of the shows varies widely, in this case because they were preserved on glass transmission discs, "air checks," simply for FCC records, never thought to have future economic or artistic value.
Secondly, the Welles' radio shows were an influence on Hollywood. You will notice that films were made from a number of these properties after Welles had presented them on Radio. David O. Selznick was constantly firing off memos about Welles' shows, and hauling his line producers and writers in to listen to Welles' programs. They were natural scenarios, from which screenwriters sometimes worked. In fact, a lot of Welles' own subsequent film work and ideas can be traced to what he produced in the radio medium.
You should not neglect his early work, such as his "Les Miserables" serial, "Hamlet," "Twelfth Night." For many Americans, these were the first dramatizations of such works they had ever experienced. And his 1937 narration on Archibald McLeish's "The Fall of the City" was a landmark. In these programs, he often worked with other important directors, such as Irving Reis, William N. Robson, and Norman Corwin, who are looked upon now as among the best the medium ever produced.
Welles' later work was notable for several things he did on Suspense, most notably "The Hitchhiker," "Donovan's Brain," and "The Most Dangerous Game." He also championed Civil Rights and Jazz in his Almanac series, and the controversial "His Honor the Mayor."
Aside from his debates and speeches for FDR (which I have never seen preserved), and his War Bond work, the "Political and Patriotic Welles" was most evident in his two shows on the atom bomb and the end of World War II, written by Norman Corwin, the latter (with Olivia DeHaviland) being a bit more of a pat on the back for the Atom Bomb.
I also like his Mercury Summer Theater productions of "The Apple Tree" and "Abednego the Slave."
Welles did a half hour adaptation of "The Heart of Darkness" for the series This Is My Best, which does not have some of the technical problems which mar the Mercury Theater version.
Hope this helps.