The NES version of Rampart is one of my favorite games of all time. Arcade not so much, (Curse you, never-fully-functioning-trackballs.)
As far as SNES, we used to get down on some Gemfire; It's a semi-clunky troop commanding RPG kind of thing.
Claymates is a pretty odd one; it's a fun platformer where you change into different animals throughout levels to solve puzzles and advance.
Battletoads & Double Dragon is probably overrated, but you mentioned two-player and I couldn't resist throwing it out there. It's too fun to play with a bro by your side. (And not quite as completely impossible as the other Battletoads games.)
Also in the "probably not that under-appreciated" category, but also another two-player banger is Micro Machines. This one can kill a whole day pretty quickly, especially if you have people rotating in & out on controllers.