What is the easiest language to build Windows services in?
Easiest in this case would be defined as least amount of code, and lowest point of entry into the language
For me, and I've only tried it a couple of ways, Visual Studio and C# was the easiest.
Visual Studio did all of the necessary service plumbing setup for the framework and I found C# very easy to learn, migrating from VB6, VB.NET and C.