Obviously, I'm referring to the 16-bit era here.
If I were being "objective," I'd pick Spriggan, as it's the most well-rounded effort of the ones listed. But my personal favorite is Blazing Lazers. Yeah, it earns points for having one of the sweetest shooter soundtracks and being a nostalgic marvel. But I maintain that its last few levels are more intense and exciting than any other stretch of vertical shooting that can be found on old-school platforms.
Here are some quick thoughts on the others:
MUSHA - The first four stages are great, especially the ravine and storm sections. The last three are pretty boring, especially the revolting "field" level. Following suit, the rock music makes an impression early on but degenerates into mush later.
Robo Aleste - I actually haven't played this one. I've heard good things and I'd probably like it, but I'm not enticed enough to go ahead and purchase a Sega CD, which has no other titles that interest me.
Space Megaforce - This is a horrible game with excruciatingly long (not to mention easy) levels. It focuses on typical SNES technological goofiness, preferring to dabble in spinning space stations and the like rather than in impressive bosses or frantic action. It also features some of the most laughable shooter concepts (scaling planets?) and awful music (save for stage 4).
Spriggan - Again, this is the most well-rounded 16-bit Compile vert. The "make your own weapon" system allows you to create and tote some awesome armaments, the graphics are very good (especially in the lava stage), the music fits the action well (particularly the creepy tune in the "bug/worm" level), the action is steady, and the bosses are quite huge.
Sylphia - It feels a little different, but yes, it's a Compile shooter, and it features reliable Compile-style weaponry. It's a very good game with a cool cast of mythological enemies and a great techno soundtrack. The graphics for the backgrounds could have been a lot better, though.
