Ahhhhhh... that game was too Cool for School (TM)
You could drive a tank, with the machine gun attached to its axle; and so you have to manuever back and forward to aim it, heh. It flashed red when it was about to blow up, if I remember correctly. And could only sustain three hits. I think?
The Metal Slug series for PS2 was one of the best purchases I have ever made. It was like, five smackaroos. Though it did make my PS2 go CRAZYHOT...
For that matter, any game which requires you to go into a Zen like No-Mind consciousness in order to complete it is my very super special comrade for eternity. Oldschool Retro Mention to Gradius V, R-Type Delta and that 1945 WWII arcade game for the PS2.
Oh, and Contra - the game which is as Gods amongst mortals.
*Takes a bow, and offers a burning, severed limb in sacrifice*
*Adds Metal Slug to underground pantheon of seisure inducing retrogames*