You may get a thousand answers to this question...each longhouse has its advantages, and every family has different needs and preferences. What is most important to you? A balcony? A view? Easy access to the TTC, or the main pool, or the boat dock?
We stayed in Rarotonga last summer, first floor, with a patio and easy access to the quiet pool. Not the most gorgeous garden view on property, but lovely, with REALLY great location - steps to GCH and bus stop. We had requested this building because we were travelling with people with mobility issues, so we thought it was most centrally located.
Rapa Nui, Tahiti and Tokelau have more bathroom space (some with double sinks) and balconies on the second floor. All the other longhouses have no balconies on the second floor. Rapa Nui and Tahiti are close to the TTC, but further from the GCH and main pool. Many families adore Samoa, especially facing the Volcano pool, and many rooms in Samoa and Tokelau have oblique views of the castle. I could go on and on...I'm sure others will add more about their personal experiences.
HTH
Aloha!
ETA: Keep in mind that you can request a location, but it cannot be guaranteed. Keep an open mind, and no matter what room you get at the Poly, there will be plenty to love about it.