My vote for the most iconic radio voice is Gerald Mohr “Phillip Marlowe”, who also played roles on numerous radio shows. Even though his stature matched his voice (unlike portly Bill Conrad), Mohr made the big breakthrough in movies.
Conrad had a deep voice, but on the radio it always sounded a little forced or artificial. The network execs didn’t let any of the radio cast make the transition to the TV version. That had to hurt, financially too.