What are you buying for $18,000? (if its a car you may encounter some trouble) Are you sure you can get a card with that much credit available?

I'd definitely go for a Starwood Amex but I know that Amex may not be an option for you and there is a annual fee.

The only Visa / MC cards that don't charge an annual fee pay miles at 1 for every $2 so they're unattractive for your purchase.

I don't think your question has an obvious or easy answer. Pick what ever airline you want the miles in and go for that one.

PS - you can still get the 20,000 UA sign up miles if you want.