I'm all for a minimalistic shooter, so I rate RSG as one of the best I ever touched. One thing I absolutely loved about the game is that it does a slightly different take on the shmup genre, I can't point out exactly what.
When I played it for the first time, back in 2000, I had one of those "gaming moments", that get imprinted in your brain for ages to come. Controls visuals and sound seemed to work damn well together in this game, and it sucked me in, that I played all way to dawn when I first got it. Even if I were to die a lot in some areas, it was not bothersome or tiresome, and I liked that.
As mentoned before, RSG puts makes you feel totally responsible when you die, and I freaking loved that. It was not that damn enemy that gets confudes with the scenery, or a bullet that killed me because the game wanted so, or that damn bullet that is WAY faster than the bunch, or because I was in the wrong side of the screen and the boss threw so many bullets at me that not even a grain of sand cound make it between them. No. There is always a way on the levels and bullet patterns.
RSG was the first shmup where I got really "in the zone", like in Tetris. I can't do the same with R-type, as example.
And the added bonus that RSG has a story of sorts, and the ingenuous way it's told (the first level is level 3, level 2 is a flash back, and so on) are really welcome.