The most generous airlines in fulfilling frequent-flier seats.

Frequent fliers wishing to snatch a free ticket using their standard mileage points have various luck depending on which airlines they are trying to use.
A recent study of availability of free seats by a consulting firm, showed that when it comes to accommodating such requests, Southwest Airlines and Alaska Air were the most generous with 99.3% and 75% fulfilment respectively.
At the bottom of the list, were US Airways and Delta Air Lines with accommodations of 10.7 % and 12.9 % respectively.

For more, see WSJ.com