I only play NES games (well, a few PC games as well), but I can give you a list of classic killer bosses, and reply to come that have already been mentioned:
Time Lord (NES) - The big guy with the sombrero in the Wild West stage...damn thing takes like a zillion hits to come close to killing it. I'm still convinced that there *must* be a secret to killing him, other than shooting him for 10 minutes straight.
Wizards and Warriors III (NES) - In this game, there is actually a remix of the original Wizards and Warriors theme...I'm surprised nobody has tried a remix starting with this remake. But I digress...if you manage to get to Malkil, he is the ULTIMATE in hard bosses. He flies swiftly through the air, throwing a series of homing projectiles at you, and has as much life as you. You basically have to trade hits and hope you get lucky once in a blue moon.
Double Dragon III (NES) - Because of emulators, I was able to save right before the final fight against Princess. That is the only way I was able practice beating her...she is fast, VERY strong, and has scores of cheap moves. Heck, all she has to do is raise her arm, and you go flying across the screen...no way of blocking it, either.
Final Fantasy 3j (NES) - The Cloud of Darkness can be an easy battle...if you're leveled up enough. But just try beating her with an underleveled party, and it takes FOREVER if you even manage to stay alive.
