Maboo, at least in the arcade version. All Maboo does is run toward you to try to kill you. You can even kill him with enough shots before he gets to you. It's easier with the machine gun.
To a lesser extent, Gimdo in the arcade version of Rolling Thunder 2.
Awesome Music: The first stage in the original. Rolling Thunder 2 (Arcade) and 3 (Genesis) had some excellent music too.
In the original game, the panthers and the yellow-skinned Bloga mutants are hard to hit, even when ducking. There's also the firemen that split into four upon death.
Rolling Thunder 3 had female maskers who attack with kicks and backflip through your shots. Fortunately, they only appear in one level.
Goddamned Bats: All three games have them. They're not as bad in the third game, since you can fire at an angle upward with your pistol. In the other games though...
Good Bad Bug: In the original arcade game, you can walk away a certain distance from an ammo closet and return to it to get more ammo. This is helpful in the case of the machine gun, which makes it a Game-Breaker. Just don't waste too much time, though...
Harsher in Hindsight: In round nine of the third game, Geldra terrorists hijack the passenger flight Jay is on, prompting him to stop them. It only took eight years after the game's release for an actual terrorist group to successfully attempt the same thing, with devastating results.
Polished Port: The Genesis port of Rolling Thunder 2 is pretty solid, adds a Password Save mechanic, and introduces some exclusive stages with special weapons not found anywhere else.
Capcom's Code Name: Viper was basically a ripoff of the original Rolling Thunder, but had you rescuing hostages like in the first Shinobi game.
There's also Rough Ranger by South Korean developer SunA, which rips off the original Rolling Thunder right down to the sprites. It even had a 2 player co-op mode predating Rolling Thunder 2 by 2 years.
