Uh... Metal Slug, but barely, for the vehicles and bosses, and gorgeous animation. Contra 1, 3, and 4 are freaking amazing though. I wasn't as into Contra on the PlayStation when I was fighting talking dogs and plant people though... one of the biggest fridge-nukes in gaming history if you ask me.
EDIT: Oh yeah, also Metal Slug for the guy's voice saying "ROCKET LAWNCHAIR!" It never gets old.