Its well known that Nintendo is not doing well this gen, Wii U will be their worst selling console ever and while 3DS is selling OK while its selling around 2x worse than previous handheld, but Nintendo still keep making great games which some of them are best in whole franchaises and that are selling great.

But I think probably biggest win for Nintendo this gen will be Splatoon. Totaly new IP that is great, fresh, fun, very popular wide world and that is selling phenomenal, and in Japan is even becoming some kind of cult with Splatoon manga drawings, musik, toys and Amiibos..

Splatoon is becoming huge new Nintendo IP, sequel is almost certain for NX, literally will become Mario Kart or Smash Bros. of shooters, basicly third huge IP that will sit along side with Mario Kart and Smash Bros. Really huge win for Nintendo.

For those who dont know, Splatoon was sold more than 1m in Japan alone, and WW numbers currently are around 3m, that is huge number for instal base of around 11.5m and totally new IP.

Agree.

It's definitely the single best thing they've done all generation. Too bad that isn't saying much lol jk.

Hands down. Splatoon was something that just came out of left field and kind of showed up all of us really. And not just Splatoon but I feel Nintendo is also benefiting in other areas as well. In the RPG realm with Fire Emblem being on the upswing and cementing itself more so as a mainstream RPG (in Japan primarily). And broadening their portfolio with an open world game in XCX.
Also they seem to be getting their advertising in order (at least NOA is). If you told me a year ago that I would end up seeing ads for XCX I would have called you crazy. Or that I would see Splatoon and SMM advertised as often as I did.

Taking all of these changes into account, I can't wait to see what they have in store going forward.

Well we really cant consider 3DS for big win if it sells LT 2x worse than previous handheld.

Even though the Wii U is a failure, it isn't a total failure. There were some diamonds in the rough that came from the system, and Splatoon is the biggest one.
It has become yet another one of Nintendo's biggest IPs that can make a lot money, sell hardware (Japan is up YoY compared to last year.), and will ultimately serve to greatly benefit them in the long haul. Same goes for the revitalized Fire Emblem, which has gone from the brink of death to one of the biggest franchises on their portfolio. And I am crossing my fingers, hoping and praying, that Xenoblade Chronicles X's sales figures are accurate, even if they're just a little bit off, I'll be ok with that, because it'll still be doing much better than the original and be on track to being one their best selling Wii U titles (I know that's not saying much, but still). Because, what happened to Fire Emblem, as I mentioned earlier, I'm hoping will happen to Xenoblade as well.

Maybe not. But that isn't because the 3DS was a loss, far from it. But rather, the DS was the biggest win Nintendo ever had in their company history, although you could make a case for the Wii.

I wrote in OP why is big win, "literally will become Mario Kart or Smash Bros. of shooters, basicly third huge IP that will sit along side with Mario Kart and Smash Bros."

I think Amiibo may be a bigger win for Nintendo this gen, at least on the financial side. I mean, it's probably not a big win for gamers due to all of the supply issues, but it is definitely a big part of why Nintendo has been somewhat profitable this generation.

In terms of quality software, though, I agree that Splatoon is very near the top of that list. So I guess overall, considering all factors, Splatoon is probably the biggest win for Nintendo this gen. It was also the first game to force IGN to change their score, so that has to stand for something!

I would argue that amiibos were more important than Splatoon, business-wise, but as games go, yes Splatoon is the uncontested winner