Really? (also got all three)
When MUSHA Aleste appeared on the MD it was amazing.
Definately not overrated by any means and how any shmup fan could say that, well, it's quite suprising. It's still inexpensive compared to some less impressive titles, even for the JPN version and a must own if you have the means to play it.
I've never got on with the CD cousin Denin Aleste, mainly because of it's lack of decent weapons and slower pace.
Spriggan wins out for me because of the amount and variety of weapons, picking up the wrong colour doesn't render you powerless either. Also, the different colour graphical effects for 'smart bomb' explosions shows the thought put in by the programmers. Size of enemies and other graphics are also larger than in the other 2 games (which seems to be the case when comparing like for like MD to PCE games) and the most important - it plays better to me.
Best vertical shooter on PCE? it's a close call.