SkyTeam has very good carriers. As carriers join SkyTeam they improve in quality.

Korean Airlines is underrated and just like Korea itself from automobile manufacturing to electronics and beyond will get even better. Korean Airlines has vastly improved over years past.

The chinese carriers like China Eastern and China Southern are getting better as well.

One can do nicely staying in the Delta alliance over time.

No single alliance will have every route in the world nonstop. However Delta's Skyteam alliance is very good.

There is no one perfect alliance to get you on the best carriers at all time. It doesn't exist.

If you must have the best airlines at all times, you will have to be enrolled in multiple alliances and FF programs. Simple as that.