I have the latest 13" MBP (for work), a 12" Macbook (personal computer), and both iPad Pros (because I'm a nerd).
The 13" MBP's speakers are actually just terrible. I would never recommend anyone use those for listening to anything, haha.
I think all 3 others have pretty good speakers. The 9.7" is much better than the 13" Macbook Pro, but they're not really as good as the 12.9" iPP or 12" MB, which are much more similar in quality.
For those two, I actually slightly prefer the 12.9" iPP over the Macbook. The 12.9" speakers seem to me to be louder and clearer than the Macbook. Also, it fills up the room more than the 12" since they aren't as directional (they seem to emit in all directions from the speakers, whereas the laptop really just seems to emit sound up from the keyboard).
I sometimes set up music in the kitchen while I cook and clean the house. There's a pretty clear difference between when I'm using the 12" Macbook and the iPad. But for regular usage (if you're just sitting in front of them), they're both equally great.