They should first go for Capcom, or some small studios, like Next Level Games.
Buying Capcom, would give them control over another of the Videogame Series titans in Japan, since they would hold Monster Hunter in their grasps. A revival of Mega-Man could also occur under their hand, and bring back old players and should get some good sales. Though only getting the IPs, could also be a good idea, though seeing how they are short in employees for development, it would be better to buy the company as a whole.
Next Level Games, even thouh a small studio have proven its worth, and seeing how they have stated that they will only work with Nintendo from now on, thanks to how Nintendo treats them, making them permanently part of the Nintendo family would be a good idea, specially since they have work in some sports games, so they can bring Nintendo some good Sports game.
Unless they can make Sonic good again, Nintendo shouldn't touch SEGA, simply negotiate with them, and buy Atlus from them.
Level 5 should also been bought, even if it simply is, to make sure that Yokai Watch, which seems to be the new big series in Japan, doesn't expand to other consoles, unless they can work something and only get the IP.
COMG guide to points: 1 point = Raging. 2 points = Beasting. 3 points = Tearing it up. 4 points = Berzerk. 7 points = Rampage. 12 points = Burst. 15 points = god mode. 20+ points = DIVIDING BY ZERO!!! 40+ points = Youkai (originally Pokemon).
-1 = Negabeasting. -5= NegaRampage. -10 = NegaBurst