The Classic. Just like Doom for first person shooters, R-Type was the first and most fearsome side scrolling shoot'em up and remains as tough today.
Complete with that oh so eerie boss music (do do do doooo, Do Do Do DOOOOO!!) plus all the kickass power ups (onions rings, mulligrubs, bouncey beam) everythings here in its original form, so whats not to love?