Bilstein B4 - factory replacement and feel for your original dampers. Vehicle will damp as it did when new.
Bilstein B6/HD - meant for a much sportier and stiffer feel, compatible with both factory springs and mild lowering springs. Obvious difference in ride quality between B4 and B6 for obvious difference in handling.
Bilstein B8 - the same sportier feel as the B6/HD, but typically with a shorter stroke and meant for more extreme lowering springs. Not recommended for stock springs.