Gigawing, Gigawing 2 and Mars Matrix are all developed by the same softwarehosuse: Takumi. Capcom only published the games. (That's why Gigawing and Mars Matrix are rather similar in style)
I have never played any of those games, but they are always mentioned as good ones.
Guwange is not that old (1999, most games mentioned in this thread are older than this). I LOVE Guwange, it's IMO, the best shmup made by Cave.