Tough call, this one. LETHAL WEAPON's a sleek, armour-piercing bullet of a movie, and while Shane's dialogue is great ("All dressed up and no one to blow." Heh, nice one, Danny), Dick Donner's direction, the Gibson-Glover chemistry and the overall Joel Silver aesthetic all contribute to its quality.
THE LAST BOY SCOUT has my undying love for Joe Hallenbeck's sarcasm in the face of his best friend fucking his old lady ("Sure, it was an accident, Mike. You tripped and accidentally stuck your dick in my wife. 'Sorry, Mrs H, guess this just isn't my week!'") and the way he makes good on his "If you touch me again, I'll kill ya" threat to that jerkoff Chet. It might also be one of the last great mainstream smoking movies - everyone who fires up a butt looks cool doing it.
THE LONG KISS GOODNIGHT has Bierko. End of story. Okay, it also has Brian Cox berating his sister about the way her dog constantly licks its asshole: "Anything in there is either gone for good or there to stay."
My thoughts on KISS KISS, BANG BANG can be found in that movie's thread in the Focused Film Discussion forum. Suffice to say it's pretty damn excellent, with Black displaying a strong directorial talent to accompany his screenwriting skills.
Might have to go with BOY SCOUT, which has aged like a fine wine as the years have passed. But GOODNIGHT is good stuff, and KK, BB is gonna win friends and influence people big-time.