Ready to redeem your frequent-flier miles and reward points for a plane ticket? You'll probably get a seat on Southwest, but good luck with Delta, at least at the basic redemption level.

Southwest Airlines has the best availability of award seats among U.S. airlines on travel dates this year between April and October, according to an annual survey by IdeaWorks Co., a travel consultancy that made 6,680 queries on 23 airline websites testing availability. The worst among U.S. airlines: Delta Air Lines and US Airways. Those two...