Even for a city so associated with sprawl, Los Angeles's relationship with towers is weird. In 1905, the City Council passed an ordinance banning buildings taller than 150 feet—City Hall, built in 1928, was one of few exceptions and the tallest tower in town until 1968—that stayed in place until it was repealed by voters in 1957. By then, the city was well into a period of car-driven sprawl, and that decentralization (coupled with parking requirements that usually translated into big parking lots) kept the city pretty blunted. In 1968, Union Bank Plaza (built on top of a parking structure, natch) became the first tower built on the redeveloped Bunker Hill and the new tallest building in Los Angeles, which set off what Matthew Au at KCET calls an "urban arms-race."
611 Place stole the "tallest" title in 1969, then gave it up to what's now called City National Plaza in 1972, which gave it up to the Aon Center in 1974. In 1989, the Library/US Bank Tower became the tallest tower in LA and the tallest tower west of the Mississippi (but flat-topped, thanks to a 1974 ordinance requiring helipads on all high-rises), titles it still holds to this day. But not for long: The spire-topped Wilshire Grand, under construction now, will set a new height record when it's finished in 2017 (it'll also be the first tower since 1974 with a non-flat roof).
For Machine Project's awesome Field Guide to LA Architecture (more here and here), artist Kamau Patton chartered a helicopter for a flyover tour of all the "tallest" buildings LA's ever had. Au also has much more on each of those buildings and their architecture over at KCET.