The best language to code it in is the one you are best at.
And if you don't know a programming language yet, you should first start learning one, before you want to write a botnet...
And if you already know a programming language, you possibly should ask a more specific question, so we know what you exactly want to know. Right now one could write a whole book about your question (also look in the ebook section, afaik there are one or two books about botnets...).