The irem classic Image Fight is my alltime favorite in this genre. It starts out simple but has a steep learning curve. They put some thought in the enemy spawning points and as well as in the enemy behaviour. Level design is decent and the boss fights are pretty clever. The last levels are really, really hard but fair. Also the way you pick up weapons, how they work and how they are arranged around your ship as shields is made very good. You can purchase it from dotemu.com with the irem arcade hits collection but I can't recommend this bundle since they messed up the sound.
Check out this youtube playthrough: