Depends on the service you use. EXO usually has the best quality, but it doesn't work on all channels and VOD (depending on provider)
I would set it as "EXO" and if a channel doesn't work, change it from the top popdown menu to "ijk player". This also applies to VOD content saying " no file" or something like that. Change to IJK and it should work. However, after you change it, youre using that media player always. Once you finish watching something that requires IJK, switch it back to "Exo".
If lots if channels/VOD stuff doesn't work with EXO, just set it to IJK and leave it there forever.
Cheers