I posted this the other day, but I think it answers this question. Survival situations are not fun, so you want to avoid them in the first place (or at least make them as short and easy as possible), so activate this basic survival skill before you even leave on your trip:
Be healthy: Being injured, tired, sick, addicted, or in bad shape greatly increases your chances of winding up in a survival situation. Take care of your body and mind and they will take care of you! This also goes for any mode of transportation or even tools. Make sure everything you need in the wilderness is properly maintained and in good shape.
Find a buddy: The wilderness can be a very fun place, and learning about survival is more fun with friends. Plus having a buddy along with you can get you out of a lot of the trouble you may find yourself in.
Train: To quote the Navy SEALs, "The more you sweat in training, the less you bleed in combat." I know this doesn't really answer OP's "one skill" question, but I think it is still important. Just make sure you are up to date on all of your activities that you are planning to do in the wilderness. I see a lot of people who haven't swam in years just diving into the middle of a river, I see people who have never been on a mountain bike attempting pretty advanced courses. Whatever you plan on doing — make sure you know how to do it well. When you are ready for more than one survival skill, this concept directly applies to all the survival skills that the others are posting on here, and all the advanced skill you will come across. Never stop learning! Never stop training!
Research: Make sure you research your route and destination and avoid anything that looks dangerous. What's the weather going to be like? What is the temperature during the day/night? Are there flash flood, landslide, wildfire, or avalanche warnings? How are the roads? How are the trails? Is there cell phone service? If you were to get lost, what is the direction and distance to the nearest place to get help? What are the major obstacles and landmarks in the area? What are the common injuries in that area? Are there trouble spots where Search and Rescue are commonly called to? Etc.
Plan: Now that you know what you are getting into — make a plan. When are you leaving? What route are you taking? Are you making any stops? How long will you be at those stops? Is there an alternate route in case something changes? When will you reach your destination? How long will you be at your destination? When are you leaving? Etc.
Pack: Now you know exactly when, where, and what you will be doing, choose and pack your gear — and then check it twice. At bare minimum, pack for the activity you are doing (swimming gear, hiking clothes, etc.) and a basic replacement if your gear gets lost, wet, or for unforeseen changes like broken down cars or surprise weather, etc. And then pack gear for the worst case scenario — your survival gear (first aid, shelter, water, signaling, etc.). I won't get too into that, as gear changes depending on your research and planning (see above).
Tell someone: Now that you are ready to go, share your plan with a person/persons that you can trust. Let them know where/when/what you are doing, how you are getting there, who you will be with, what you look like/are wearing/driving/etc., any stops or likely detours that you might make, and when/how you will check in, and what to do if you don't check in. Then stick to that plan! These are the people that will activate Search and Rescue when you don't check in. The faster SAR are activated, and the more accurate information they are armed with, the quicker you will be found and treated.