I've always thought that Vaillant Arena, the home of HC Davos, is a beautiful structure:
So too is the Scandinavium in Gothenburg, Sweden. A dead ringer for the old Capital Centre; or rather, the Capital Centre was a dead ringer for the Scandinavium, as the latter was built about four years before.
One of the most unique had to be the Pittsburgh Civic Arena (better known to people under the age of 20 as Mellon Arena). SkyDome (now Rogers Centre) has always purported to have "the world's first fully-retractable roof", which is complete BS. The Pittsburgh Civic Arena was the first, almost 30 years before SkyDome, and it opened a LOT more than SkyDome's roof:
Unfortunately as the facility became more decrepit the hydraulic system used to open the roof didn't work anymore, and I don't think it was ever opened in the last 15-20 years of the building's existence.