Many, many opinions on this... One of the mods posted a while back some info showing the average reef temp throughout the world was 81 degrees. Tidal areas around Fiji and some other tropical islands along with tidal pools can reach into the 90's during low tide. The GBR was 74 degrees when I was diving back in August.
General consensous is anywhere from 78 - 82 and to try to avoid wild temp fluctuations.