I don't know if it's the best, but I use PDF-Writer.NET, for which I paid $89. I have used it in several production applications. I like it because it's easy to set up and use, and it doesn't require a lot of coding, which makes it easier for new developers to ramp up on it.
Before that I was hacking together PDFs using an open source library and the Acrobat interop DLLs. That was rough.
