6. Mushihimesama Futari: Ultra Mode
Release date: 2006
The most difficult mode of Mushihimesama Futari is recognized as one of the tougheset single-loop shooters of all time, and for good reason. The sequel to Cave's Mushihimesama is rife with insects, crabs, and other strange creatures that want you dead. Dense enemy fire, increased bullet speed, and incredibly tough boss fights are appended to the final boss if you can somehow manage to reach the end without burning up a continue.