Search and Destroy!!
Personally, it's the answer to the both Life Force and Commando on the Nintendo Entertainment System. You can upgrade your weapon like in Life Force, and it's a top down shooter like Commando. So inbetween, it's a cross between those games, which put into one, makes a perfect top down shooter.
The difficulty in this game is correct, and evened to the bone, and you can continue right where you left off, instead of back at the beginning like in most games.
So if you're collecting Genesis games, you should pick this game up, not tomorrow, not this Christmas, but right NOW.