Among the options you have mentioned, Starwood (as mentioned above) and Diners Club are the best. You can transfer your existing Diners Club reward points to United, typically $1000 charged to Diners is equivalent to 1000 UA miles.

Also you can use Diners Club to transfer AA and DL miles to UA but you get a 50% penalty.

[This message has been edited by PG (edited 08-13-2001).]