I'm planning a project which will consist of a Windows Server Application programmed in .Net/C# and Clients programmed in Silverlight/C#, Windows Forms/C# and a MacClient programmed in Cocoa. My Question is, which Webservice technology will be the best for the communication between the Clients and the Server and is the easiest to program in all of those technologies? I've no experience in Webservices and since Time is running short I hope to get some opinions of developers who worked in such a kind of heterogeneous project.
The current technology used to develop web services on .NET is WCF. For interoperability with non .NET clients you should consider using basicHttpBinding endpoint. You could even expose multiple endpoints with different bindings, for example expose interoperable endpoint for non .NET clients and some fast binding for .NET clients. Here's a nice article on MSDN covering the performance of the different bindings. Given those keywords you might checkout the tutorials.
On the back end, you will definitely want to run Windows Communication Foundation, using the WSHttpBinding or the BasicHttpBinding depending on your needs.
This will make it easier to have Windows Forms and Silverlight clients.
Also, because using WCF with those bindings conforms to established standards, you should be able to access the services from almost any other environment - there should be tools that you can just point it to your metadata endpoint and it will generate proxies for you.
Another option is to use WCF to create a REST service (with JSON encoding most likely). WCF does help you a little bit here, but if this is the design you want to use, then you will want to look at ASP.NET MVC on the back end as well, as it makes creating these kind of services very, very easy.
However, when using REST services, there isn't a service description through something like WSDL, so you will have to generate the proxies to call your services by hand (at least, in environments outside of .NET).