genre: let's fucking go
examples are wideranging and include blaster master, which opens with a ramp-up timed to fit the colossally badass cinematic of your tank revving up and taking off, which you see exactly once at the outset of the game, to let you know to buckle the fuck up. game of the year award for "best first 10 seconds if you're 10 years old"
Verytex is a mediocre shmup with an absolute masterpiece of a stage 1 theme because Hitoshi Sakimoto was incapable of producing anything but masterpieces
(fun fact: he tended to use only PSG sound effects in his Genesis games so he could dedicate all the FM channels to the soundtrack, which is perhaps a decision i don't 100% agree with because the PSG is pretty lousy)
but tyrian 2000's stage 1 theme is extremely Curiously Uplifting
that said, when i think about stage 1 shmup themes the single one that immediately comes to mind is Zero Wing's