There are LOTS of different schools of thought on this, since every longhouse is near one convenience but far from another. You might want to check out tikiman's site, he has the pros and cons of each. My personal experiences have been with Rapa Nui and Rarotonga. I loved the locations of both. I was lucky enough to get a garden rather than parking lot view in both longhouses.
Rapa Nui has its own very convenient parking lot, and is the closest longhouse to the TTC. It was also very quiet and peaceful. It's not as close to the main pool or the Great Ceremonial House than other longhouses, but nothing is very far from anything else at the Poly anyway.
Rarotonga is centrally located, so it's pretty close to everything. What I liked about this longhouse is that it's right next to the Great Ceremonial House, as well as the laundry facilities. I had a ground-floor room with a patio, so I was able to leave my room right through the sliding door to go out to the pool.
I really don't think you can go wrong with any of them, but there's no real best or worst longhouse.