the shoot'em up system is the PC-Engine. No other system has as many shoot'em ups as this little beast.
And in 2nd comes the MSX, with another huge library of this genre.
Saturn, Dreamcast and Mega-Drive are the other contenders
I think James Pond 2, Ruff 'n' Tumble, Arabian Nights and Soccer Kid are games that are in the same league of quality that is both Sonic and Mario (Though both James Pond 2 and Soccer Kid ara available on other systems). I'd even say that Ruff 'n' Tumble is better than most Mario games... (I am more a Sonic man than Mario)
And there's NOTHING like Benefactor on consoles, which is one the finest platform games ever.