One thing that skews Mediabase is probably an uneven number of stations by format. Also remember that the CHR stations have a very high rotation rate so those songs have an advantage. But back to the question...
As far as an artist that gets more play on more stations than any other, it would probably be Mariah Carey. She clears all current CHRs, Rhymics, Urbans, Urban ACs, Dance, and Hot ACs. Her older stuff gets played on AC, Soft AC, and some Variety Hits stations. I bet you can hear Mariah on 93.3, 94.1, 94.9, 95.7, 98.7, 100.7, 105.5, and maybe 101.5 as they dip into early 90's up tempo hits. That's a total of 7 if not 8 FM Tampa Bay radio stations. And if you wanna count the WTMP simulcast, that would add another.
As far as an actual song that gets played on more stations than any other.... I'd lean towards something by Boston or bands like Styx, Loverboy, REO Speedwagon, or the Cars. It's gotta be hard enough for 102.5 yet a big enough hit from the 80's for a station like Mix 100.7. "Don't Look Back" or "More Than A Feeling" would fly on 100.7, 101.5, 102.5, 104.7, and 107.3. That's a total of 5.