I admittedly played only a few shooters in my lifetime, but one game that got me totally addicted was Space Megaforce for the SNES. There was apparently something about that game that made me keep coming back for more.
It wasn't until recently when I found out the game was made by Toho, the same studio that created Godzilla. Awesome.

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AKA Super Aleste and part of Compile's legendary series. Just like Robo Aleste and what might be my personal favorite shmup, Musha Aleste:
That one has a quality about it that kept me hooked to the end. Gate of Thunder is also awesome. And Thunderforce IV. There are many good shmups to choose from.
The worst? I don't know. BlaZeon is pretty weak aside from the cool gimmick of turning your ship into a mecha. Hellfire looks nice but is insanely difficult.
Jam it back in, in the dark.