Based on my travel plans this year I´d say: Air Rarotonga

Seriously, I wouldn´t mind exchanging SQ for CX, other than that BA, QF and LAN all have a large network which I wouldn´t mind taking advantage of.
However, I don´t expect any of the large OW or ST members to "change sides" to *A anytime soon. There are still a few decent non-alliance members out there (and a whole lot more of mediocre ones) which may join *A, but I wouldn´t necessarily say I´m too thrilled about that prospect.