In its heyday in the mid-sixties thru the early 70's WABC, NY (Lodi, NJ) probably had the biggest signal in the USA..At least was 'heard' by more folks due to the population density of the area east of the Mississippi. The Morning guy, Herb Oscar Anderson, once said, "Good Morning to all our friends listening in Denmark!'
Some others that had HUGE signals --before the noise floor rose to the present 'crud level' .....WLS (still huge), CKLW..WKBW, WCBS..WWVA, WHO, WSB, KOMA (DJ Bobby Scott used to ID late at night..."KOMA, Oklahoma City, Oklahoma--One of America's TWO great radio stations"...the other one, The Big KB (WKBW) on 1520! KB could be heard in EAST OKC...but everything west to Hawaii KOMA was boss. KOB in A, NM ran on a temp license for years ..directional WEST protecting WABC on 770...when they closed down on Saturday nights WABC could be heard in Los Angeles! Moving west KFI, KCBS, KCBQ, KFRC were all dwarfs to 93 KHJ! At the same time XERB in TJ was running about 100kw along with some other Mexico giants...XERF and Stereo XTRA...Wolfman Jack was on XERB (from HIS studio in Hollywood). For a short time XEROK was running a MILLION WATTS across the Rio Grand into the southwest. They could be heard on 800 in the daytime in NM, AZ and points west. Phoenix, AZ talker, KTAR was heard in LA as well as a small-time country station, KHAT.
Who had the largest radio audience worldwide....in the Cold War years...no question Radio Moscow dominated the planet...they could be heard on every band 24-7 worldwide!
Who dominates today...most 'listeners' in the USA...probably WINS or WCBS New York...in the rest of the USA...its market to market..but NY IS the number ONE radio market in the country..so most listeners per 1/4 hour...I'm sure some FM outlets in LA and Chicago are close in numbers but 1010 WINS or 880 WCBS rule the pack.