I think the UA app is pretty decent, but as a Canadian I tried to make a purchase on it and it wouldn't accept a Canadian card/address.
The AC app works fine for me, but admittedly it's full of IT holes.
Copa app sucks, sorry, I love the airline but the app sucks.
Avianca app is fairly limited, but for what it DOES do, it's fine.
AA app is pretty decent, but I've only used it once or twice.
LOT app isn't great. ANA app is a bit confusing, it doesn't seem to ever go really English.