I like this question and am a couple years late joining this party, but will chime in late anyway. How many species of freshwater otter are in zoos in North America? San Diego Zoo used to have a Eurasian river otter, but I think that it croaked.
I know of Asian small-clawed, spot-necked, North American river, and giant. Are there any others?
Is there any consensus on where the model exhibits for these species are?
For giant otters it seems like the huge exhibit at Moody Gardens in Texas gets excellent reviews.
For North American river otters there seem to be multiple excellent exhibits.
For Asian small-clawed is the National Zoo considered to be the premiere exhibit in terms of space, enrichment, and viewing?
For African spot-necked otters I agree with mstickmanp that San Diego has an excellent combined Allen's swamp monkey-otter complex. The otters have abundant space to romp, swim, and interact with the monkeys. The Reid Park Zoo also has a decent spot-necked otter exhibit. Are there any other good spot-necked otter exhibits in the U.S.?