For the purposes of this thread lets assume the NX is a handheld, I don't wanna go into why I think it is here.  What would be the one game you'd want it to launch with?  

My pick is Monster Hunter 5.  MH4U was my first MH game and it is by far my favorite 3DS game, that's right I pick it over KH:DDD.  The improvements in MH4 completely change the way you play the game, I couldn't even imagine playing a MH without climbing or mounting, can't wait to see what Capcom does with MH5.

I say Pokemon cause Pokemon is generally the biggest system seller for Nintendo handhelds and is loved by Japan and Western people alike! Plus the games are great!

If I had to choose semi-realistically, I would go with Fire Emblem 15!

If I could choose any game I would like to see TWEWY 2

I gotta say Flappy Bird man! Sell em like hot cakes!
Jk gotta go with Yuri on this one. Pokemonz!

you've mentioned NX as handheld. about to vote for Mario. as a Super Mario Galaxy 3 as a launch title would be a great launch title. Since it is handheld, i've also opted for Pokemon.

Metroid. Pokemon, no. Assuming NX is the console. Pokemon Battle Revolution was a POS because it was rushed out. If it's the handheld Fire Emblem.

Paper Mario and a Full HD 3D pokemon that has full MP support. If it launched with that it would be the first Nintendo console I had owned in over 10 years.

I like how the OP says "Lets assume its a handheld" but people still respond with it "being a console" statements

A new Yoshi game. Or maybe a sequel to Eternal Darkness ( If they don't make a Wii U version.)

A Mario Galaxy 1+2 remaster would be awesome as well.

I'd say about 50:50.. :P