I played the game.
But I find it hard to really think of only one.
When I go hunting I often have a small hatchet in my day pak, With a Cabelas tamahawk on order with a carrying case ot cut down on weight and make it easier to carry. The little pack hatchet I have does not have the leverage that the longer handle of the hatchet does and I can trim the wood handle to fit in my day pack. That usually carries a spare folder or a light weight fixed blade like the Cold steel Lite pendelton hunter.
I carry my $ in my front pocket with a Gerber Air Ranger being used as a money clip. I use it as a tool almost everyday to cut and strip wire, use the scales as a file, and have used it to jimmy locks and have stuck it in door jams to make sure I do not get locked on a roof again. (I am a boiler salesman and often have to getinto boiler roooms, and connect controllers or add jumpers when needed) I was once on the roof of a bank and was not allowed ot have my cell phone in the bank, The keys they handed me were to get out but the lock the other way to get in was a different key. I was on the roof for three hours before I could climb down to a level where i could get the attention of someone walking in and asked if they would get the manager to get me off the roof and then had ot climb back up 2 stories to the roof, -Yes I stick the knife in the door jams so the can not close.
I have a thick bladed fixed blade I use as a hunting knife that does alot. and on my Game bags and shooting vests I have a Dozier design Ka Bar folder that are sharp, strong, and easy to resharpen and they are always there when I go shoot sporting clays or wing hunting. I find it hard to think I only have one to choose from but it was a good game.
I have a buddy that carries a Gurka Kukri that acts as hatchet, machette, and knife. I think it too big, but have used it to make firewood and clear shooting lanes when setting up a blind. It worked but I think a good keen hatchet works better. I also have one of those bush blades that has the brush hook on the back that was issued in WWII. Mine does not have the hand guard. I used it once when duck hunting to add some cover and again find my Fiscars hatchet to be more effecient.
I think a better question would be "what bladed implaments do you carry in the field and why?".