I think as a series Smash has the best chance of beating Halo 3 first week, it always seems to be the most hyped up game for Nintendo systems, but I think it won't do it with only like 25 million users when it comes out. I suppose there is a chance that Pokemon next year could do it if it comes out at the holidays. First pokemon console game ever and Switch user base will be over 40 million, I could maybe see close to 1 in 10 Switch owners buying Pokemon first week.

I'd give it to Pokemon Sun and Moon. If Europe hadn't been released a week later it would have demolished it. Also as Pokemon S&M was available digitally at launch? (Don't think Halo 3 was till later?) Even without EU it probably beat it.

Anything with very front-loaded sales is the best bet.

Smash Ultimate will be the best selling exclusive first week at launch, i'm calling +6 million sold.

Didn’t God of War on PS4 already beat Halo 3?

Edit: Nevermind, I just looked up the FW sales of Halo 3 and it was close to 4m while God of War was a little bit over 3m.

Are you high?

Pokemon X/Y is listed as 4.2m (first week) on VGC.
http://www.vgchartz.com/game/71449/pokemon-xy/?region=All

Im thinking more low

But GoW did 3.1 mil in THREEE DAYS; whereas, Halo 3 did 3.8mil in one week. I think an extra 4 more days of sales for GoW would’ve given Halo 3 a run for its money.  

Diablo 3 sold 6.3 million copies in one week. Granted, the game is on consoles, but it was ported later, and the entirety of those fw sales was on PC, as it was PC exclusive at launch and for the year following.