The answer to this question really depends on your goals and available resources, so I'll give a few answers depending on some possible scenarios.
Assuming, for some reason, you can only (or only want to) do bodyweight exercises and are stuck indoors:
Any/all of the following:
Assuming you want to improve leg strength, over-all fitness and body composition, but can't use a gym
See above, but also:
Assuming you want increased fitness, leg strength, body composition, etcetera, but don't fancy using a gym or think a barbell will hurt your back
Just go to the gym, try hard and always try to improve your form, knowledge etc, and remember that it will get easier and less 'intimidating', if that's your issue.
Barbell squats, deadlifts and power cleans can't really be replaced by bodyweight exercises.
Having said that, sprinting is king (but I may be slightly biased).