The best April Fool pranks that I know of were done by KROQ.
- One year they said they were "Groovy Hits of the Super 70s" and recreated the sound of a 70s-era Top 40, complete with jingles, cheesey jocks, songs like "One Bad Apple" and "Afternoon Delight," and weather reports for "the Beach Cities." I still remember turning on 106.7 and hearing American Pie come out. (This was back in the early 90s, when there had not yet been a 70s revival, Disco Saturday Night, or things like I Love the 70s on VH1. So it was a real shock!) The jocks participating (and doing great 70s voices in 2 hour shifts) were Tami Heidi, Freddy Snakesin (who was called Freddy Foreskin for the day), Scott Mason, John Frost, the Poorman, and Jed the Fish. Not sure if Kevin and Bean participated.
- Another year they declared "Pirate Radio" was back.
- And yet another year they said KROQ had gone Spanish. They were called "KROQ en Esapgnol."
Unfortunately I think the tradition pretty much has died on KROQ.