I'd probably go with Taihiti. I haven't heard anything about Cook Islands, Samoa or Tonga being big travel destinations.
You may be able to get some good deals for Fiji at the moment as they have had political difficulties recently and are keen to get the tourists back.
Either way I'm sure they all have sunshine, sand and water so you can't go wrong.
Also don't forget that Australia is also an island in the Sth Pacific.