What is the highest rank a WWII sniper can get?

Almost impossible to answer properly without the question being more specific. In the British army they would be able to go through the usual rank progression, although it could well be that by the time they got to Sergeant they would be employed elsewhere that the front line as snipers, after all a sniper is just a specially trained infantryman and not a seperate unit.

Im talking about the US army sniper. My bad about not informing you better.

I would think that it wouldn't be too different .

Same answer as above. A Sniper is simply a specially trained Infantryman. Sniper is just another skill identifier. He would progress through the ranks as any other soldier and could go as high as his performance and potential carried him.