I used PdfSharp. It’s free, open source and quite convenient to use, but I can’t say whether it is the best or not, because I haven’t really used anything else.
Update:
I’m not sure when or if the license changed for the iText# library, but it is licensed under AGPL which means it must be licensed if included with a closed-source product. The question does not (currently) require free or open-source libraries. One should always investigate the license type of any library used in a project.
I have used iText# with success in .NET C# 3.5; it is a port of the open source Java library for PDF generation and it’s free.
There is a NuGet package available for iTextSharp version 5 and the official developer documentation, as well as C# examples, can be found at itextpdf.com