No particular order overall (can't bother choosing only five, sorry!):
Ys III/I & II/IV in that order - each OST's amazing
Dracula X
Nexzr
Aldynes
Rayxanber II/III
GunHed
Devil Crash
Soldier series (i.e. 3 different games)
Gate of Thunder
To a lesser extent: Lords of Thunder (reminds me too much of Joey Tafolla or Joey MacAlpine music, i.e. great shredders of the 80s), Spriggan 1 and 2, Sylphia and first stage music of Terra Cresta II (love that track!).
These rank among my favourite OST's ever. Btw, I have absolutely no nostalgia whatsoever for these titles, having grown up in a household with NES + SNES. That says a lot about the quality of the soundtracks found on the system.