I am kind of worried (as always with me) about the 3rd party support we will get with the Wii U. For example, the Wii was more powerful than the Gamecube, Xbox or PS2 and we still didnt get much good games from 3rd parties because all of the big titles were sent to the HD twins instead. Every big project (assasins creed, call of duty, battlefield, portal, tekken, soul calibur, skyrim, final fantasy, etc) were designed to use the capabilities on the 360PS3 and were never release (or were released with tacked on controls or inferior features) than their counterparts.

Capcom saw a great succes with the Resident Evil 4 remake for the wii (it was the definitive version for the majority of people), and what did they do? they dont even care in bringing RE 5 and released the on rails stuff.
The same can be said about all the other 3rd parties.

I am afraid that with this new generation 3rd parties wont care to release their games to the Wii U :( even thouugh its more powerfull than the HD twins, and as the wii it will get really bad games that wont max out the system.

The reason the Wii didn't get many of those games is because the Wii used a a graphics system called a TEV as compared to the HD consoles which used programmable shaders.  Because of this, developers basically had to rebuild their entire game just to put it on Wii.  Because the Wii U and the PS4/Next X will all use modern programmable shaders, porting between consoles will be far easier and less costly than downporting to Wii was last generation.

While I get what you're trying to say, you've made a mistake.

A killer app doesn't have to sell a large number of copies in order to be a killer app. In fact, if a game sells 50,000 copies, and every one of those copies is bought by someone who didn't previously own the system, and they bought the system specifically for that game, then it's a killer app... for those people.

"Killer app" is one of those subjective terms - different games are killer apps for different people.

So how are you mesuring how many systems a game sells? 

The increase in weekly sales of the console the week the game is released?

That how we have always done it ("PSP increased its sales thanks to the release of Monster Hunter", etc.)

As JEMC has already mentioned before me, the easiest way to identify a killer app is the impact that it has on system sales around the time that it releases. Unfortunately, it can become a problem when multiple significant games release in the same week. There is also the fact that a game can simply be the straw that sells the camel's back (heh) - a game that is just enough to make the system worth purchasing, but which isn't really the one that sold the system.

Typically, we won't actually try to identify killer apps that sell 50,000 copies, because there's no reliable way to identify them. But Smash Bros is much easier to test. We'll just look at USA numbers, because you really need to look at each one at time of launch, and I can't be bothered seeking the other numbers.

In the week that Smash Bros launched, the Wii saw more than double the sales it had seen a week earlier, up 81,012. The only other Wii title of any note to release that week was House of the Dead 2&3 Return, which sold just 11,834 copies in that week... far too few to explain the boost in Wii sales. Wii sales continued to be very strong (slightly stronger in the second week, in fact), with no major Wii release. All up, it looks like the release of Smash Bros sold the Wii to somewhere around 180,000 people in the first two weeks.

So for USA

pre Brawl 67,160, Week 1  148,172 (+121%), Week 2 216,423 (+46%). Arround a 230k boost using the pre week as baseline

pre Kart 80,223, Week 1  333,130 (+315%), Week 2 123,983 (-63%) Arround 216k boost using the pre week as baseline

pre NSMB Wii 206,768, Week 1 294,086 (+42%), Week 2 521,456 (+77%) Arround 197k boost using the pre week as baseline

pre Wii Fit 148,172, Week 1 308,891 (+210%), Week 2 150,985,  Arround 14k boost using the pre week as baseline

not sure how you would do Wii Sports, and Kart was a few weeks after Brawl and Wii Fit a couple weeks after and NSMB was at Xmas and had supply issues if I remember as did Brawl. Then there is the factor of esisting install base, as the install base grows the number of people that can be sold on the console by a single game decreases. Then there is the long term effects thing like Wii Fit likely sold a lot of consoles far beyond it's first week as word of mouth spread it's first week was just over a 10th of Brawl's but ended up selling almost twice as much. Plus as you said other titles launching at the same time.

So not a really reliable metric I would say, tho by that metric it does suggest that Brawl was the biggest killer app over the shot term.

There was a reason why I used the phrase "in the first two weeks". The other killer apps likely did better over time for the system, with Brawl being more up-front. The key is that generally, if it shows a significant boost to system sales on release, it's a good sign that it's a killer app. The extent of the boost isn't necessarily proportional to the total, though.

The week of release doesn't matter ...

Games like Nintendogs, Brain Training, and Wii Fit had an unusually large impact on the sales of Nintendo's hardware by reaching out to unconventional demographics that didn't currently buy a lot of gaming systems. While these games probably sold far more systems than most platforms killer apps combined, the sales happend over months as new people were exposed to these games and bought systems for themselves.

The best measure for whether a game is a killer app is its total game sales because there is a portion of any game's userbase that bought the system for that game; and this will likely fall in the 5% to 20% range for most games. When you start dealing with games that sold tens of millions of copies of games for a system it is likely that they were directly responsible for the sales of millions of pieces of hardware for that system, and would clearly be a killer app.