Definitely their exclusive series, there's pretty much no other reason to buy a Nintendo system at this point. For the last couple of generations, their consoles have been far weaker than the competition's, eliminating most chances of acquiring third party content, and their gimmicks (like motion controls and dual-screen play) have been a hit or miss kinda deal. The only thing that's left is their first party games, which while they may be created with like a fraction of the budget of most AAA titles in other systems (and it shows in several kind of ways), they more than make up for it with what's still some of the best designed (and polished) high-quality content. So good job, Nintendo.
Another reason's starting to pop up, though, and that's local multiplayer. Seems like other consoles are starting to abandon it, which is a ****ing shame. Right now, beyond indies and a few random small games on the online store, there's barely any major release on PS4 I'm interested in that has local multiplayer. Thankfully, that's gonna change soon, but it's a real problem for now. All retail games I've bought so far are single-player only but offer some sort form of online multiplayer.
I mean seriously, even racing games no longer have local multiplayer, that would have been ****ing unacceptable a couple of generations ago. It still is for me, though, there's some kind of games that are ****ing worthless if I can't play them with friends, and racing games are definitely one of those. I mean what's next, fighting games becoming online-only?
In the meantime, more and more Nintendo games are getting local multiplayer. Even leaving aside the obvious stuff like Smash and Mario Kart, most of their freaking platformers can be played with up to four people, and platformers were basically a single-player-only genre back in the day.