Agreed, the soundtrack is one of the things that keeps me playing Gradius games. I haven't played any of the Parodius games, as I don't tend to enjoy the "paro-shmups", with Fantasy Zone being an exception. I'll probably give Twinbee a shot here soon, as well.
Salamander/Life-Force is great. I have the NES version, and the PCE version on my Iphone. I enjoy the NES version more, mostly because it has the same power-up system as Gradius.
If I'm not mistaken, it seems Konami's Gradius games for the NES typically surpass all other ports, with the original Gradius being a possible exception...not that it's bad or anything, it's just somewhat basic as compared to Salamander, Gradius II, etc. I like the PCE version best, I'd say.