If economy is not a problem,the large iPad Pro is the best,but the iPad Air 2 is also great as video or music streaming device. A hint: use the iPad Air 2,with HDMI adapter and cable,connected to a TV set,works great for me,and the adapter with cable is not that expensive.
I have the 2017 iPad, the sound is good enough for me. I haven't turned it up to it's loudest settings. But if you are using the iPad as a TV replacement, I'd probably go with the Pro since there are more speakers.