easy, Zelda I could honestly say i have not been dissapointed by any game in that series, at least the ones i have played

this is tough, down to mario and pokemon for me

might have to give the slightest of edges to mario because of mario kart

Too hard to choose.
Smash Bros., Mario kart, Zelda, 2d Mario, 3d mario, DKC, Fire Emblem, and Paper Mario are all so great.
Now that I think about, the 3ds has almost all of these franchises coming out(2d Mario and 3d mario are kind of mixed though) so I guess my distaste for the way the hardware looks is the only thing keeping me away from it...

Since Metroid Prime is my favourite game ever I'll have to go with Metroid.

I'd go with first Zelda, with Nintendo Wars as a close second.

Mario, closely followed by Zelda. Yes, I'm going for the predictable choices.

Mario
Metroid
Smash Bros
Mario Kart
Zelda

Far and away
1.Smash Bros

Followed by
2.Endless Ocean (although that may not count)
3.Fire Emblem
4.Kirby
5.Mario Kart

This thought has never crossed my mind to have to choose between Nintendo's IP, but I have to admit this is extremely difficult.
If you put a gun to my head though (the way I ran the scenario in my head numerous times), I will have to choose Zelda with Metroid a close second.

Nice to see EO get a mention on this thread! I hope Nintendo find ways to keep it fresh and interesting!

I also have to agree with Rol in giving honorable mention to Wario Land, and add my own honorable mention which is Pilot Wings.