Mine is most likely Mario Kart Wii I got my WIi when it came out. I constantly have taken it to friends houses for Wii party nights. New Super Mario Bros is my favorite game, but due to my friends loving Mario Kart and it being out longer, it has the most playtime.

What say you?

As a household, that's easy- Fire Emblem: Radiant Dawn. We've gotten almost 200 hours out of that game thus far. Second place goes to FF4: The After Years. I don't remember what came after that, but I do know that it is sub-100 hours. I would guess FE for me personally, also, but it's tougher to get individual playtimes...

Fire Emblem Radiant Dawn, and it's not close.

NSMBW me and a friend played it till we finished it 100%

Rune Factory Frontier, but Super Smash Bros. Brawl is a close second.

Super Smash Bros Brawl, almost 310 hours.

Brawl, probably followed by Mario Kart.

Mario Kart Wii is easily my most played Wii game at about 300 hours.  There's not a single game I've gotten over 100 hours on outside of Mario Kart.

It would likely be more if I didn't take a hiatus.

And the Wii isn't a console for hardcore players. Of course, hardcore players only want 6-8h single player campaigns...

Super Smash Bros. Brawl followed by Resident Evil 4