Probably my favorite C# article on this site has been the C# List article. The List generic type is extremely useful: it can help in many programs. And it has been one of the most popular articles as well over the lifetime of Dot Net Perls.
The article has been updated many, many times, but some memories come to mind:
Even if it may not be the hottest new thing on the Internet these days, I think the C# List article is still my favorite piece of writing on this site. It has helped many developers figure things out in their own programs over the years.