If you put a gun to my head and asked me to pick only one exercise to do for the rest of my life, I would undoubtedly choose the squat.
The criteria I am using to answer this question are four-fold:
First, what would be the most functional movement in day-to-day life? Squatting is important for sitting in chairs, getting up from the floor, and any other movement that involves bending your ankles, knees and hips at the same time. It also improves walking gait and balance.
Second, what exercise would have the longest range of motion? The squat outshines the deadlift, the lunge, and all other lower body exercises in this category. Squatting actually increases your range of motion the more you do it, as your hips, knees, and ankles all become stronger and more mobile.
Third, how much strength can it produce? Second only to the deadlift for maximum weight that can be moved, the squat is a force to be reckoned with. Nothing builds muscle mass, bone density, and maximum strength like the squat.
Fourth, how modifiable/progress-able is the exercise? With the squat, you can take someone who can only manage to sit down with assistance and increase their loading over time to the point where they can squat to full depth with a weighted barbell on their back. I know this to be true because I have personally seen it. Furthermore, you can use box squats, high bar back squats, front squats, and overhead squats to increase the difficulty and type of demand on the body.
Are there any negatives to squatting? Only a few. One, unless you utilize an overhead squat or front squat variation, the shoulders and arms are largely unworked. Two, if you don't progress the exercise properly and focus on proper form, injury could occur, but that can happen with any exercise, so buyer beware. Three, without a barbell, it is difficult to maximally load the squat pattern. Four, it is hard (at first).
Take inventory of your exercise routine and consider adding more weighted squatting, unweighted squat holds, and anything else resembling a squat. I do weighted squats 3-4x per week. Each morning I hold a twisting squat stretch (about 30sec). I poop using a squatty-potty. I floss my teeth in a deep squat (about 1 minute). I also look for every opportunity to squat throughout the day.
If you are new to the movement, it will be uncomfortable at first, but stay with it at least 3 months. Keep in mind that everyone's squat will be a little bit different based on limb lengths and flexibility. Use a doorway or solid object as a counterweight to help you get low. Squatting will change your life.