Air France KLM’s Flying Blue program should be interesting to U.S. frequent flyers for several reasons.
They offer one-way awards. They are an American Express Membership Rewards and a Citi Thank You points transfer partner. They have far more award space on their own flights than Delta has access to. They have most of their partners available for online booking.
Unfortunately their call centers are frustrating, their website can be too (and has been known to show ‘phantom’ award availability, seats that look available but aren’t really here to book).