As I have never been able to play some of the finer imports and arcades have always sucked near me, here's 10 of my favorites.
Gairies
Axelay
Blazing Lasers
Mars Matrix
R-Type series
Psvariar Revision/Medium unit- can't remember which
Donpachi/DoDonpachi
REZ
Shikigami No Shiro
Ikaruga
These may or may not be 10 of the greatest shmups, but at the time I first played them there was something about each one that I really liked. Graphics never being a factor. To this day I can pop in a copy of any game listed and play it through and not be bored, thoroughly enjoy it.