Also worth noting:
While Top 40 did start in the places David mentions in 1954-55, L.A. was later, at least in terms of the format being on a fulltime, full market signal. The earliest Top 40 was KPOP, the daytimer at 1020 AM that is now KTNQ, David's alma mater.
KFWB was the first 24 hour station to try it. It went Top 40 on February 1, 1958.
KRLA followed in September of 1959, but it wasn't popular until 1963, when they brought in an impressive group of hired guns including Emperor Bob Hudson and Casey Kasem from KEWB in Oakland, Dave Hull from WFLA in Tampa and Dick Biondi from WLS. Later that year, they toppled KFWB. But within two years, KRLA was second and KFWB third to KHJ, which launched April 27, 1965.
KFWB had five years of dominance. KRLA two.
KHJ was number one for four years, in the top two (with KABC) for 11, and didn't lose to another Top 40 station for 14 years, despite competition along the way from KFWB, KRLA, KDAY, KGBS, KKDJ, KIQQ, KIIS-FM, KTNQ and KFI.
It might not have been pulling McLendon numbers from another era, but that's a real strong showing.