Hmmm...good question..I would have to say:
1) A Few Good Men
2) One Flew Over the Cuckoo's Nest & Batman
3) Something's Gotta Give & As Good as it Gets
Obviously I have a few ties in my opinion:)...I happen to also love love love Jack Nicholson (as you may already know), but I just couldn't choose between certain movies. The "YOU CAN'T HANDLE THE TRUTH..." in A Few Good Men still gives me chills until this day. The acting in that movie is phenomenal, but I can honestly say that Jack takes the cake. He's just such a badass!!
I also can't leave out One Flew Over the Cuckoo's Nest and Batman. I simply couldn't choose between the two. I love his "nutty" personality in the Cuckoo's Nest, and I absolutely loved the sinister yet humerous Joker that he played in Batman. (I am also looking forward to the late, great Heath Ledger's portrayal of the Joker character as well, but Jack is definitely going to be hard to beat!)
I absolutely also loved his portrayal of a bitter man with O.C.D. in As Good as it Gets. I really don't think anyone could have played that part as well as him. He made me actually believe that he had this disorder. It was extremely believable. Something's Gotta Give gets my vote simply because he found a way to show his softer side while still staying true to himself; His personality shined through even though the movie is considered a bit "mushy" (but I like it because I'm a girl!), duh!
Anywho, my honorable mentions are going to have to be The Shining and The Departed. I absolutely love both movies as well, but as far as my favorite roles that he's played I'm going to stick with my list.
I definitely agree with you as far as seeing him play a General in war film. We already know that he can pull off the part. Other than that, I feel that he has the perfect eyebrows and facial expressions to be some type of a murderer or serial killer. I mean, I just want to see him play a really evil, sinister character because I feel that he would pull it off beautifully.
Uhhh..as far as the bonus..I'm going to have to cheat and say that I read it off of someone else's answer..It's from One Flew Over the Cuckoo's Nest..there's no point in lying:) I got the right answer, right? So it really shouldn't matter where my source came from!! LOL J/K
P.S. I've never seen Chinatown..is that any good? I've heard of it, but never really heard of any reviews...just curious I guess. Let me know whenever you get a chance por favor. I hope I helped with your question. Peace and Love!