the guy with a gun at the end of double dragon that kill you with one shot
the guy with the machine gun that keeps jumping all over the stage in batletoads
the last enemy from shinobi 2
alma from ninja gaiden black puts a really good challenge in both forms
and probably it does not counts as a boss but the last stage of driver for the ps1 was just crazy.
@eab
actually a finish the game in hard, is the only way to get the real ending.
My definition of fanboy: An idiot that misses 2/3 of the fun.