Which animal is least well suited to being kept in a zoo?

Those that travel a far larger distance than would be even vaguley realistic in a zoo. Or those that live in such large groups it would be too crowded in a zoo.
Great white sharks don't do well in captivity at all - I think 44 days is the longest ever a great white has ever been in capitvity before dying or being released

No animal is happy in a zoo. Least happy would be large animals that live in a tight knit group like elephants and that travel hundred of miles regularly.

Blue whales.
Humboldt squid.

These sea creatures would require impractically large enclosures.

The natural roamers and migraters...like elephants that travel in herds hundreds of miles. Or herds of antelopes.

The tiny areas afforded captured migration animals cause them to be nervous and stir crazy.