I would like to point out a particular favorite of mine.
Touhou 9: Phantasmagoria of Flower View
It's different from the rest of the Touhou games, in that it's a one-on-one competitive STG. I'll try to explain the basic rules of how it works.
- Killing an enemy will spawn a bullet on the opponet's screen
- Killing spirits (The colorful bird-like things) will send unique (character-specific) bullets to the opponent's screen.
- The small bullets can be destroyed with enemies' explosions. Doing this will make them re-appear on the opponent's screen (Bullets can be sent back and forth forever in this way)
- Players build up "Gauge" over time. Spending this will launch a special attack at the opponent, as well as clear out any nearby bullets.
- The number of bullets keeps increasing until one player loses all their health. The last player standing is the winner.
Gameplay Video: https://www.youtube.com/watch?v=P71FMPIcD9c
"Never argue with stupid people. They will drag you down to their level and then beat you with experience."