By "hazardous" here I mean, most least likely to survive the course of the war. Let's leave out support positions like cooks and clerks. Was it better to be assigned to a tank crew, than to infantry? Artillery? What would be my best chance of seeing combat, but also coming home?
Infantryman; without a doubt. This should be no contest.
The exact numbers for actually becoming a fatality (note; fatality, not casualty) are about 1 in 122 for the US Military if we go off easily available data on the net; but that statistic would probably be of little comfort if you were a Rifleman.
First off, you're most likely a draftee, and training and qualification may have been shoddy as the Army is hurting to get you to the front as quickly as possible. When you arrive, you'll arrive without a familiar face in sight; as the Joes you've trained with are all shipped off where they're needed at a depot upon arrival in France. You're already not in a good place psychologically as a replacement. You're off balance, everyone's a stranger, you're most likely underprepared, and the vets that could prepare you are all suffering from the emotional and physical strain of sustained combat. Nevermind the fact that you are currently replacing old comrades who have been maimed, wounded or killed.
A US Rifle Division is around 14k men. It wasn't uncommon to have a unit within that structure suffer 100 percent casualties; meaning that between Wounded in Action and Killed in Action they could see a full rotation of manpower. A few examples;
The 36th ID suffered around 13.8k WIA during its fighting across Italy and Southern France.
The 1st ID, one of the longest fighting units in the US Army, had a rather cynical saying by the time they were in NW Europe: "The US Army consists of the 1st ID and one million fucking replacements." Like the 36th, they too would suffer over their paper strength in Wounded and Killed.
Your best chance of 'surviving' WWII would probably come from being an Airfield technician or something Stateside; nothing was truly safe for any army - its why its war, if we choose to ignore support units, then your life is potentially in mortal peril at all times. Artillery, especially in mobile operations, routinely found itself on its own and fighting for its life; the British often used 25Pdrs as emergency Anti-Tank weaponry throughout their Mediterranean fighting, and the US Field Artillery involved in Cobra, especially the self-propelled units assigned to CCA and CCB of the 2nd Armored, found themselves dueling unsupported with Germans on both sides of the Vire River - as the Germans tried to escape the pockets being formed by the American breakthrough, or tried to fight their way towards their encircled comrades.
TL:DR the question breaks our rules. No, we're not removing it, but read our rules please!
To those who have reported this thread; you're right. The question does break our rules. Specifically, it is a "poll-type" question - not a good name for it, but normally you can tell by the way it has "most/least" in the title.
This thread as well has gathered some excellent replies; I am loathe to remove it as it has some good discussion going on. So it stays -but please read the rules before you post, as next time we will simply remove it.
A related question: if I had been drafted during WW2, how could I have ended up as a cook or clerk? I presume health/injuries may have had a lot to do with it?