IMO, the big difference in image quality in iPad displays is laminated display vs not laminated display. The rest is nice but not as big of a difference.
So, if you want something that's cheap but better screen than the new iPads (2017/2018), get an iPad Air 2.
If you don't mind spending a little more cash and want better sound (quite nice imho) and the TruTone display (not a big deal imho), get an iPad Pro 9.7.
The 12.9 might be worth it if you don't mind the money and the drop in portability.
The 10.5 probably won't be worth it purely for watching videos - I've got one, and coming from an Air 2, the screen wasn't much of an improvement to me. The main advantage here are the speakers, but you can get those with a cheaper 9.7 Pro too.