I broke my back brake lever, and I want to use only one brake, which should I use the front or the back?
Common consensus is that the front brake is capable of providing a lot more stopping force compared to the back brake.
So if it was a choice between front and back, I'd choose the front brake.
In reality, your bike needs some maintenance. If you're competent with tools then you can do this yourself, otherwise you're up for shop labour costs.
I would strongly suggest replacing both brake levers, even though only one is broken. Its probable they are the same brand and the other one could suffer the same breakage.
If it's to get you home or to the bike shop, use the one that works and ride cautiously. You may have to walk down some things. Trying to fiddle with it at the side of the road or trail is risky - you don't want to lose a screw. If you're considering long term use it's a bad idea to run only one brake in most situations, especially as you know the ones you've got are prone to failure.
As you're taking about full suspension I assume you mainly use it off road. I'm not a mountain biking expert by any means, but on loose surfaces you don't have much stopping power from the back as a side is both easy and ineffective. On the other hand a light touch on the back brake is very useful, and overheating can be an issue on a single brake. On road you have the same effect to a lesser extent but the consequences of your second brake failing can be more severe (an uncontrolled meeting with fast-moving traffic).
I would generally agree with what has been posted, but I will add a caveat about where you are riding, and respectfully disagree with user32345. On loose surfaces, the rear brake is usually better, and on ice definitely so, but on pavement the front brake is the only one that makes sense. In hard braking the weight shifts significantly forward, and it then takes very little rear braking for the rear wheel to break contact with the surface, and when that happens with a rear brake only you have no brakes at all. A need for hard braking usually means that something bad is about to happen. On loose ground you can juke the bike and slide selectively to shed speed, but that isn't likely to work on pavement unless you are an expert.
I fully agree with user32345 that the most sensible thing is to get both brakes in working order, and soon.
To simply answer the question. "Which brake should you use if you only have ONE:" YOU SHOULD USE THE BACK BRAKE. Its easier to fade in and out and you will have better control while braking, especially on rough terrain/ loose surfaces.
With that cleared you should get both your brakes back in order as fast as possible!