I was going to say 3DS, but then I saw that the 3DS only managed 1 week over a million last year and seeing how that system is already noticably down this year, I doubt the 3DS will manage another million week.

Other than that the PS4 seems to be the only other candidate to maybe do it, depends on stock and how well Sony manages to keep up the momentum though.

PS4 will have big weeks this holiday. I'd say it'll have atleast a couple weeks above 1, or even 2 million.

For me PS4 will sold 1 milion, but in week before Christmas of 2016-2017.
I Think only 3DS this year will sold 1 milion.
For XO and Wii U i think never, but 500/600k for XO and 700/800k for Wii U is not possible, for me XO will sold like or more of Wii U in generaztion, but on Christmas Nintendo have boots of sold much best.
For Vita think 250/300k, maximum 400,000.

2 million seems very, very ambitious. To say it's a feat not even the Wii acheived and the DS did only once (I believe). 

If Smash Bros 3Ds comes out fairly late in the year then it could push the 3DS to have a million week

3DS is Nintendo holiday machine isn't it? Perhaps PS4 doesn't hold enough stock or keep demand going, but both have the possibility.

PS4 and 3DS...
XB1 closish - WiiU and Vita half

How many times have ps2 hit 2M for a week?

The 3DS has the best chance. I don't see any others selling a million a week, unless there's some really nice bundle or must have game out.