Ikaruga Classic Co-op
The Matrix has 28 shoot 'em ups on file for the Sega Dreamcast, released between 1999–2015. 12 of them support multiplayer, including 5 with co-op. They're ranked below by legacy score, so the games that shaped the genre on the Dreamcast come first, with lesser-known picks further down.
Shoot 'em ups (shmups) put you in a ship facing waves of enemies and bullets. Runs are short and intense, and the goal is to survive, collect power-ups and push your high score.
Typical commitment: quick session (under an hour, or endless arcade fun). Tip: Focus on your ship's small hitbox rather than the flood of bullets, and learn which power-ups suit each stage. Most shmups are designed to be replayed many times.