After switching from KHJ-FM to KRTH, it's been:
Oldies from 1972 to 1976. The switch from Drake-Chenault's "Solid Gold Rock n' Roll" to KRTH's own in-house oldies (focusing on 1955-63) saw the ratings jump from 15th place with a 2.5 in the fall of 1971 to a tie (with KGFJ) for fourth place and a 4.3 in the fall of 1972 (the rating period began a month after the format launch). But the buzz didn't last, with KRTH falling to 13th with a 2.7 in 1973, improving somewhat to 11th with a 3.1 in 1974, falling again to 15th with a 2.4 in 1975 and then hitting rock bottom in the fall of 1976 at 19th with a 1.8.
Adult Contemporary 1977 to 1986. This was the way Adult Contemporary was in the late 60s and through the 70s and what it's returned to now...all but the hardest songs from the current charts, mixed with gold that goes further back in time than the Top 40 stations. A massive improvement...10th place with a 3.4. Fall '78 saw 11th place with a 3.0, Fall '79 8th place with a 3.8, Fall '80 9th place with a 3.4, 1981 9th place with a 3.1, 1982 12th place with a 3.2, 1983 12th with a 2.8, 1984 12 with a 2.6 and 1985 tied for 11th with a 2.9.
Oldies from 1986 to 2005. KRTH added Beatles, Beach Boys, Motown and other 60s music to its mix for its second attempt at Oldies. It started strong in 1986 with a 3.8, which it held in 1987, but erosion began after that, with a 3.5 in 1988, a 2.9 in 1989, and a 1.9 in 1990. KRTH then hired Mike Phillips as PD and Bill Drake as its consultant and it rebounded with a 3.8 in 1991. Robert W. Morgan and The Real Don Steele joined in 1992, which saw a jump to a 4.0. The numbers stayed within half a point of that until Morgan and Steele left the air due to illness in 1997 and died in 1998, at which point the numbers floated down to about a 3. When it slipped below that in '04/'05 is when the writing was on the wall.
Classic Hits from 2005-Present.
Its most sustained success is in its current incarnation. Second place would probably be the six years under Mike Phillips and Bill Drake before Morgan and Steele died. Then would be the first five years as an adult contemporary, followed by its original oldies format.