I think comparing both IIM Ahmedabad and IIM Calcutta is doing injustice to these colleges. Both the IIMs are excellent at their own level.
If you consider the rankings then IIM Ahmedabad tips the list and IIM Calcutta comes next. But do rankings really matter that much? It might be possible that a certain college is on top of the list but inconvenient for you and a certain college is second in the list and is best suited for you.
I have seen friends who have rejected offers from IIM Ahmedabad and went for IIM Calcutta because it suited their needs better.
Some special features of IIM Calcutta that IIM Ahmedabad does not have are:
A finance lab with a Bloomberg terminal
Top notch finance faculty
CEMS-MiM programme (Masters in Management programme ranked 7th in the world)
So yes IIM Ahmedabad is the best IIM but IIM Calcutta is also not an ordinary one. Choose the one that suits your needs best rather than which ranks the most.