For me it's none other than Nintendo's:
This little game is so damn cool. You can jump between control of anything and anyone on the battlefield, soldiers, tanks, choppers, ships, while giving orders to your troops.
I really don't think Nintendo has taken as much advantage of it as they could but then again their online system kinda sucked.
But now with Wii U, they can take this game to the heights it was meant to be. The online, if done right, would be awesome! Plus, with the controller the game not only has more gameplay possibilities but you can have a nice little local multi-player.
Obviously this game would never be considered 'hardcore'. Too cutesy-like, I guess, but I'm sure if Nintendo puts more effort into it it could become fairly popular with the 'core' and 'casual' market.
So, what game franchise do you think has alot more potential than is taken advantage of?