
Bubble Man/Wood Man (Mega Man 2)

Mega Man super happy fun time disco hour yeah!
Mega Man had so many wacko enemies that it’s hard to pick just one. After 8 traditional Mega Man games, there are not a lot of excuses that could be made for the fact that two of the absolute worst robots appeared in only the second game. What could be more useless than fighting people with leaves, like “Wood Man” (Which is kind of a funny name in and of itself)? The answer would be bubbles. That’s right, Bubble Man seriously tries to kill you with bubbles, which you in turn go on to try and murder other robot men with in later stages. Great job with that one, Capcom!
The Kelbeross (Ninja Gaiden)

It's actually "Kerberos", but you know how the Japanese are with the letters R and L...
Ninja Gaiden, while not as difficult as Contra, was a bit of a controller smasher as well. To this day, the series is known for its high level of difficulty and totally awesome bosses. To me, the most memorable boss in the first (and second) game was not one of the several human assassins that you fight early on, but the nasty pair of dog-like monsters that guarded the gate to the final, supernatural-themed level. The Kelboross look like the aborted fetuses of an English Mastiff on HGH, only slightly more animated.