Just remember that if you go AA, you will not earn miles/Tier points on BA flights between the UK and US. You can earn miles/Tier points on any other BA flights. This is a competition ruling.

SkyTeam includes Air France, KLM, Alitalia, Czech, Aeroflot. So I am not sure why you are trying about another air alliance.

One to look out for is BMI a member of Star Alliance. There is currently 9,000 miles on offer when you join up and
a) will status match in a flash
b) has rather low threshold for status
Get you UA, SQ, LH, etc.

You will have Star Alliance Gold immediately and it should be easy to retain you if fly UA or LH transatlantic and UA or SQ trans Pacific