We need to know where you based, which places you redeem to, class of service you prefer, what you expect out of a FFP (i.e. upgrades on paid Y tickets, C or Y awards, elite status recognition).

But in general Star has more members, while the main partner of AA charges YQ (BA). AA charges less miles for some places, while UA charges less for others.

Do not use DL, it is a living nightmare when it comes to redeeming miles.