American, US Airways, Delta, United, JetBlue, Spirit, and Frontier all fly to Costa Rica from their various hubs. The last two are probably not options for converting points from these programs you mention. Aeromexico (connect in Mexico City), Copa (connect in Panama City [Panama]), and Avianca (connect in San Salvador) also fly here to CR, they have alliances with various U.S. airlines.
People have their favorites. I always fly American because that's the frequent-flyer program I'm invested with. I think a lot will depend on availability. August is only a couple of months away.