Tahiti and Tokelau would be great for 5 people, including 3 adults, because the rooms are amongst the largest (Rapa Nui also has the larger rooms). Tokelau is a LITTLE more centrally located, and Tahiti is a LITTLE closer to the TTC, but they are adjacent to eachother, so nothing is really far from anything (IMO). Tokelau is about half-way between the GCH and the TTC, and very close to the bus stop (but I prefer using the buses at the TTC for DHS and AK because they are direct, no stopping at other resorts on the way). Tahiti and Rapa Nui have double sinks, none of the other longhouses do.
If being VERY close to the GCH and the MAIN pool is important, I recommend Rarotonga, Niue or Samoa, as these are the buildings that are closest to the GCH and the volcano, without being too far from the bus stop or the TTC. These buildings have rooms that are slightly smaller than the ones in Tahiti and Tokelau, but they are still amongst the biggest standard rooms on WDW property.
Looking at the map may be deceptive, and may give the impression that the property is very spread out, but IMHO it is a very compact resort. It took my family about 5 minutes to walk from the TTC to the GCH.
Hope this helps. Enjoy the Poly!