How I'd vote on this depends on what we're talking about:
(a) Best zoo for otter exhibits; OR
(b) Best single otter exhibit.
If it's best zoo, then Jacksonville and Omaha would be my top nominees, because both of them have 3 different underwater-viewing exhibits, with Omaha showing off 3 different species. I'd probably lean slightly towards Jacksonville, however, as they have giant otters, and the river otter exhibit in their children's zoo is great fun for kids, with the chance to crawl underneath the otters in an acrylic tube. National Zoo is also a great zoo for otters, with 2 different nice underwater-viewing exhibits.
If it's best single exhibit, I really love Maryland, with its otter tunnel in the children's zoo. Other nice ones include Buffalo, Dallas, and Caldwell. Another unique otter experience is St. Louis' otter slide in their Children's Zoo, where kids can slide through an acrylic slide right through the otter pool.