There's a Stichodactyla sp. anemone off the Solomon Islands thats supposedly been known since the late 1800s.
Some species of Xenia apparently have a 2 year life cycle- the shortest natural life span I'm aware of for a coral.
For many massive LPS (think Montastrea or other Faviids for examples), its a tricky question to answer, and depends on your definition of "lifespan". Core samples of some colonies reveal that they are centuries old. But remember, a colony is a collection of identical clones, all of which budded from an original single polyped individual that had loooong since disappeared. Since in many species those clones arise from the original polyp splitting into multiple clones, each subsequent split contains an ever decreasing percentage of "original" tissue. So, is it still one individual? A colony? Something else? Its really tough to define in cases like that.