If we only count physical sales it's possible Splatoon will appear to sell more. But it seems Halo 5 digital sales are going to be unusually high, so I think with true sales Halo 5 will sell the most by a decent margin.
I also don't think Splatoon will get close to 5 million. If Wii U was going to get a 30 million end of life LTD then 5 million for Spaltoon would be on, but a sub 20 million Wii U, just not a big enough install base.
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