The only lens which I enjoy more on the crop 7D2 is the 24-105L. On the crop sensor the really distorted edges cannot be seen and it gives a pretty descent picture. On the 5D3 I always need to crop the edges of the pic.
99% of the time I have a 150-600mm Tamron on my 7D2 for wildlife, the other day I took the 5D3 instead with the same lens, I saw a fox at distance, an animal I rarely get a chance to photograph, I missed too many shots compared to the 10 frames per second of the 7D2 and could not get a decent close photo even out of the cropped image. The shots are OK but I know that this is where the crop sensor and speed of the 7D2 would have done a better job. Ive found the best lens on my 7D2 is my 70-200F4L as it seems they were made for each other. As for my 5DIII nothing shines on it more than my 135F2L I love, Of course wide landscapes with my 17-40L on 5D3 I cant beat with my 7D2. But like I said, different reasons to have both. 1DX is very nice but once cropped in I'm left with a very small file with wildlife shots, I tried a friends one. Happy to use both with the subjects best suited to them