I'm also fond of "Kill!" which I recently purchased. Nakadai shows less intensity but more range with his role in "Kill!"
The following reviews will simply show a picture with the movie title followed by one word that best describes (for me) what I like about that movie.
These are movies I've seen (and all of them more than once). Some might call it sacrilege that I didn't include any of the 60s/70s Zatoichi films- but I've only seen a couple, and while they were good and fun (and I hope to see the rest), they weren't necessarily my favorites. I also hear that "Three Outlaw Samurai" and "Sword of the Beast" are also good, but I have yet to (purchase and) see them.
"Sword of Doom"
"Yojimbo" (AND "Sanjuro")
"After the Rain"