What airline is the best to fly a dog from Costa Rica to USA?

I adopted a dog from Costa Rica and am trying to get him to the US. He’s a little dog but could be nearing the requirements of being too big to be under the seat. I am hoping he can fit under the seat in a carrier but need to know what airline is the best and most likely to allow a dog. Everything I’ve read has been hit or miss no matter the airline and I’m nervous to commit to one and her turned away. Any help would be greatly appreciated.