Wilhelm I von Oranien-Nassau
In Wiesbaden we have two statues of a Wilhelm I. I've already shown you Kaiser Wilhelm I. (1797-1888) a while ago, but this one is Wilhelm I. von Oranien-Nassau (1533-1584). He was also called William the Silent, was born in the House of Nassau and became Prince of Orange (Oranien) in 1544. In the Netherlands, he is also known as the "Vader des vaderlands" (Father of the fatherland) and the Dutch national anthem, the Wilhelmus, was written in his honour.
The reason, why we have his statue here, is that Wiesbaden is part of the former dukedom Nassau. Also, it is said that Wilhelm died for his Lutheran beliefs and the church behind the statue is the Marktkirche (right next to the city hall) which is Lutheran, so to honor him his statue has been set up in front of the church.
To find out more about William the Silent, check out the article at Wikipedia.