Hi Gary,
I am in two minds, I would have to say Star Alliance, for the quality of its members, they have Singapore Airlines
which I absoutly love, and are expanding their alliance very quickly. For return on money spent/miles earnt I have to say Oneworld because of American Airlines AAdvantage and their Platinum Challenge.
If you dont fly enough to reach OWS just sigin up for American Airlines Platinum Challenge, many people have been awarded AAdvantage Platinum OneWorld Sappire (OWS) with just one international full fare return trip.