The Hessen State Theatre in Wiesbaden. This theatre is subsidized by the Hessian State Gov't. Three…read morediffferent stages for Opera, Ballet, and symphonies.
Opera-wise the STW is not really in the same league as nearby Frankfurt. The productions are less grand and the level of singing talent not as consistent. It is also a much more casual affair with many students in attendance. Mid week performances you're more likely to see jeans than formal attire. On weekends and for Premiers it's a bit dressier but nothing like a major opera house.
The Opera House itself is a little over 100 years old and modeled roughly after Vienna and Prague so it has a very old-fashioned feel. It's actually a good place to go for opera newbies. Ticket prices are very reasonable,, dress-code lax, and they rely heavily on thhe standard repertoire so there's a fair chance you can see La Boheme, Tosca, or The Magic Flute.
Park in the Casino parking garage just past the Casino at the top of the Wilhelmstrasse.