Saaz is almost required for a Czeck Pils, but it does well in a German Pils too. HM and Tettnager are ones I often use in my German Pils.
On the American derivatives, ones not mentioned but worthy of using in a brew just to see are Vangaurd and Santium. Or use several in a German Pils. I have made some good ones using Vangaurd, Tettnanger, and HM or Sazz for bittering, flavor, and aroma respectively. Some are saying to make a blend, and use that blend for all additions - that is one the list to do!