IKARUGA / Treasure / Dreamcast
Treasure has been more known for their innovative and quirky side-scroller/action/platformers than anything else, but they've got a few sh'mups in their stable, and Ikaruga is probably the most renowned of them. It's the standard vertically scrolling 2D formula of one lone spaceship up against ridiculous amounts of enemies and their endless waves of projectiles, but with a new gameplay wrinkle - the ship can "switch polarity" from black to white at a tap of the button, and all the enemies you face are either one color or the other. Enemies of the same color can't hurt you with their beams and lasers (though they can still ram into you and push you into walls), but you do double damage by hitting an enemy with the opposite color's beam.
The Dreamcast version is a precise port of the original arcade release, down to still having joystick motions displayed on the Tutorial mode instead of Dreamcast pad inputs! Apparently only 50,000 of these were released, and only in Japan ... the game would later get a Gamecube re-release worldwide, but it was also kinda limited, and still sells for about $30-$100 depending on condition and completeness. Your best bet, outside of MAME or burning a CDI for this one, is to check out the Xbox Live Arcade downloadable version released a couple of years ago, which touches up the graphics a bit.
Not that they needed a whole lot, since the game is really great looking, with 3D models flying over a constantly shifting 3D landscape in the distance. The music also has a suitably epic feel for the difficult, do-or-die theme of the game.
Unfortunately, even with the neat "polarity" gimmick and the aesthetic level of quality, Ikaruga is a fairly standard mix between "bullet hell" and "memorization" style shoot-em-ups, catering to a fairly small and particular niche market (thus the likely reason for the low print runs.) The game has only five levels that last about five minutes each, but they're brutally difficult. Unless you're already a hardcore sh'mupper, or have the willingness to invest a few weeks in level memorization and playing the same two or three levels over and over again, the game quickly leaves you with little to get from it. Seriously - I'm not usually one for "spoilers", but anyone considering actually dropping money on this game needs to watch this video first, to seriously evaluate if you think you will ever really be able to complete this game. If you're not up for weeks of training to handle that, enjoy playing the same three five-minute levels over and over and over again, because that's about all you can do with the game.
Though I couldn't dig up any real cheats for the Dreamcast version, at least the small amount of unlockable content (an art gallery and sound test, among a couple other things) doesn't absolutely require you to complete the demanding challenges (such as finishing levels without getting hit and completing the whole game on one credit.) Bonus stuff is alternately unlocked simply through cumulative playing time - for every five hours, up to twenty, you get a new unlock. I don't know if the other versions work that way, and I imagine the original arcade version doesn't have this stuff.
There's a lot to appreciate here, but the short length and brutal memorization-based difficulty make it pretty much inacessible to all but a niche market.
* Gameplay Video