I have Raiden Trad and I have really enjoyed it.
Have that too. Not sure how I came across it. It's cart only and I tend not to buy unboxed games. It's quite a nice port. When I switch my Megadrive region switch to JP I get a Japanese title screen. There are quite a few early cartridges like that.
Other MD shmups I have: TF3, Twin Hawk, Hellfire, Musha (repro), Space Invaders '91, Zero Wing. All very solid games.