Thunderforce II - best shooter ever - imop. (tho not really vertical. But - Its got horizontal and top view missions)
On sega genesis,.. in mame as megaplay (but not sure if working well in mame) Also, was originally developed on sharp's x68000 computer (also known as fm-towns)... with more levels, better gfx details and sounds, much harder as well.
Top view parts are a bit bland compared to the side
scolling levels... which are jaw droppingly cool (even better the higher you get!) Sweet fast smooth play and lots of weapons (seven total capacity!). Great synth tunes and sounds.
Better than the sequals in many ways.
Very 'replayable' as well.